The thing is there are lots of other mitigating factors.

If a mother is determined, and in many cases, since they are the ones initiating, or due to financial reasons, they can and are likely to claim a pletora of reasons that can be skewed in their favour.

Things like domestic abuse, sexual assault, abandonment, lack of parental presence(fathers in society are expected to be the less available, so the default is mom is always around), and things like are often used as weapons to deny fathers equal custody.

50/50 while should be the default is still seen as a victory for men, and even in those cases, they are still expected to provide child support and sometimes alimony and if unable to, jailed.

Also a large thing to keep in mind. It is extremely easy to mention contesting cases here, something that shouldn't ever be the case. 50/50 should be the automatic default. However, the reality is most average men cannot afford a lawyer that requires a lengthy custody battle.

Women have charters, or suppor groups, hell even governmental assistance that has free legal aid provided to them. Men have access to none of those. Single mothers also have these afforded to them, including housing. Single dads do not. So guess who eventually gets seen as 'more capable of taking care of the kids?'