I think it is for a few reasons.

1.  Women are viewed as mothers.. primary caregivers... and their identity of being a mother is so wrapped up that they hold incredible ownership over their kids and that title.. while men associate their identities more with what they DO for a living.. vs who they gave birth to?

2.  The courts because of that mother bias as caregiver are often much more likely to favor the mother in custody proceedings.. putting the man in a more noncustodial role.. the part time parent.. which puts them at a disadvantage and at the mercy of their Ex'es motherly ego.

Society also tends to put the "stepping in" male as someone taking care of someone else's kids... as a positive.. while a woman who has any whiff of taking over another woman's offspring is viewed poorly.

I think traditional roles place societal pressures.  BM and DH split pretty amicably at first.  But then people started telling her that she could get more (more custody, more money, more control).  And since there's a lot of precedent out there of BM's winning she got a bit greedy and territorial.  In truth I don't think she really even wanted them 50/50.  But she felt it was an expectation and if she didn't have custody everyone would wonder why. So she fought, they ended up with 50/50 (which is what they'd agreed on at the beginning), and the relationship was ruined.  Court and fighting defined their relationship for so long that it made everyone around it miserable.

In my case, my ex and I split, did the EOWE agreement and that was pretty much it.  He's old fashioned enough that I don't think it even occurred to either of us that he'd want or get 50/50.  He didn't really enjoy them when they were tiny, so I was the primary care giver.  By the time they were old enough to be really interesting to him it was long set that he was an EOWE parent.  I do believe that if he'd had a GF or wife to back him up he might have been talked into fighting for 50/50.  Thankfully we had it all settled between ourselves before new people got involved.

Last thought - not all men had strong parental role models.  If they don't know how to be a good parent, then not only do you have resentful ex's in the picture, you also have poorly behaved children.  And if you're in a BM-centric state/country you have the looming threat of losing your children at any time.  It's a bit of a time bomb.

Thankfully there's men like you and my husband who know how to raise strong children and support their wives!  Some step families work out just fine.

Here's a good link :

https://www.psychologytoday.com/us/blog/stepmonster/201106/why-its-easie...

She makes many great points but some takeaways I found from the article were:

- According to researchers including Mavis Hetherington and Constance Ahrons, after a divorce women experience more resentment and anger, and experience it for longer, than do men, who are more likely to nurture fantasiesof reconciliation and work for "smooth sailing" with an ex-spouse.

- Stepfathers, on the other hand, have a wider berth to step back and let things develop on their own with their stepkids. As one man with a stepson told me, "I wanted my wife to be a mother to my son. I even thought it should come to her 'naturally' somehow. But I didn't feel the pressure to be that to my stepsons. They already had a dad and I was clear about that.

- Divorced and re-partnered or remarried fathers often feel fearful of incurring the anger of their ex-wives ("If she gets mad, I might never see my kids again") and of alienating their children if they say "no" or hold the kids to a high standard of behavior

Part of the answer is that there are generally way more (unrealistic) expectations towards stepmothers than stepfather.

I think men are less likely to seek out advice like this, much more likely to complain to their friends in real life.

And stepdads are heros if they just show up and have a job. The expectations for a stepdad are the same as for a regular dad. Stepmom and BM expectations are vastly different

I also don't think many men identify themselves as a "stepdad" like that's not a huge part of their identity the way anything to do with relationships and motherhood are for women.

Women have the "Evil stepmom" stereotype to deal with.

They also have the "mom" role to try to either ignore and deal with people talking about how "poor step kids" have to deal with a woman who "should" play the "mom" role, and BM hates it and tries to make your life hell because you aren't doing enough.

Or they TRY to fill the "mom role" and NEVER live up to it, but then get told they are "trying to take the mother's place" and BM hates it and makes your life hell, because you're "trying to replace her".

As a SM you also have ANOTHER situation of SDs... they are different than SSs in the way that "Daddy's little girls" can do no wrong, ESPECIALLY if you get guilty feeling dad's trying to make up for "tearing the family apart" by "not doing enough" because they got out of an unhappy situation.

In intact families, Dads are TRRADITIONALLY (I emphasize "traditionally"because it's not always the case) are background characters in the home, they "bring home the bacon" and that's acceptable (in society). So if a STEP father takes this role, he's seen as a HERO, someone who married the single mom and is now taking on a father's role... there isn't much of a "shoe to fill"... the societal bar is set low for men, always...

I mean none of this as an attack on men, it's just the way society is, it sucks, and needs to change, but how do you change society as a whole?

There are real stigmas in society that are still "shameful".  Fathers can be non-custodial parents and this is considered 'ok'. If a mother becomes non-custodial, she is seen (especially by other women) she must be mentally ill or abusive or criminal, certainly no mother would just give custody to dad so she could move for a career opportunity. Women shame women.

Step dads vs step moms... I am just supposing, I could be wrong, but step moms deal with a lot more than step dads do

Step dad's dont seem to have to battle with bio dads as much as step moms/bio moms. Women really are momma bears when it comes to their children and throw some narcissism/mental disorder/lack of moral codes in there and step mom deals with a side of bio mom that maybe step mom's hubby doesnt see in his ex wife. Women from childhood have a special communication and competition and shame system...usually men are not crushed with a look or a word, but girls/women are cruel to eachother....whereas men might go throw a few punches, shake hands, and shoot some hoops. Men are not usually viscious emotionally to other men. So, right there step dad's do not have to deal with the emotional turmoil from his step kid's bio dad. And bio dad's don't usually use their kids to manipulate their ex like women do, in order to mess with step mom's head (oh look, I can still get my exhusband to pay attention to me...intended to barb step mom).

In general, I think step dad's dodge the emotional hells and deal with setting up boundaries with step kids more. Step moms have so many expectations (they put on themselves too)... and their new husband may expect them to be mommy to their kids...or they might exclude new wife, which causes her to feel like she doesn't know a part of her husband. Maybe step dads are better at handling these kinds of things, or maybe men feel bad about reaching out for support...but I wonder if men are able to not take bio parent insults to heart like women do?  Step dads seem to be able to set boundaries with their step kids...whereas step moms often are not allowed to disciplin or set up boundaries that are respected...and yet step moms are still expected to clean up after sick step children, and take care of them...the bio dad does not usually take on the 'mom' roles after he gets divorced. Many times a divorced man with children remarries partly so his wife will be there to do the dirty work like wash the kids clothing, clean the toothpaste off the bathroom counter, etc. Women are still expected to be housewives, honestly. Step dads dont have those expectations. Just my thoughts, I could be wrong, just speaking from my experience and what I have seen...   

Rags, you were able to emphasize boundaries and rules with your step son, and you are blessed with a wife who teamed up with you, and your efforts turned out well, that is awesome :)  As a woman, I can not IMAGINE suggesting to my now ex that his kids needed to be our beck and call slaves til they saw the light. (which is genious!) Not his precious 23yo duggie dependents!

I DO believe if step parent and bio parent are rational people and want the best for the children (and are not using the children to fill some hole inside of themselves that they really should get therapy for) then they will team up (which I think is key that the couple is a team first) and have un-movable boundaries that the kids will either fall in line with or go live elsewhere. 

Rags, I am thankful that you are in this forum as an example of what spouse team work can accomplish

it is the ex wives/BMs that make it more difficult. It may be fair to say that women take longer to move on and begin new relationships. If that woman happens to be a toxic, angry, narcissistic mess then that makes the problem for the man and his new wife even worse. HCBMs never get over the fact that men dared to leave them and have moved on to a more loving and caring woman. In my case, the HCBM is still raging mad that my husband left her. After 7 years she cannot stop complaining about it and she hates me with an unwavering passion. Sure she's never said more than a single hello to me and I sure did not break up her marriage but she doesn't care. I'm a home wrecker in her eyes and she will still spread the lie to everyone. My ex husband, an abuser, while a major butthole, doesn't really interfere or badmouth my husband and me, at least not to our kids. It's sad to say but it's harder to be married to a man with children because of his ex and her toxicity. It's so much easier to be happy and get along but some women cannot and we have to, sadly, deal with them. Some of us get lucky because they alienate themselves and their toxic teen/adult children and that makes our life a hell of a lot better!!