Laws change in legislatures, not homes.
But ignoring that aspect, I agree with you, to a point. One big reason mothers get custody more often is they do more child care, especially when kids are younger. I think if everything was completely equal mothers would get custody more often for that reason.
But it's still a little suspect, at best. Not many people are comfortable with separating mothers from their children, even to live with the father, unless she is abusive, neglectful, or mentally disturbed. That includes most judges. Also, things are a bit stacked against fathers to begin with. Men work more hours outside the home (on average) than women, even ones where both have full-time jobs (I think it's an hour a day more). So men already have less time to spend with the kids. Most women want a husband who makes at least as much money as she does. And in my experience, most women don't really trust their husbands with child care as much as they do themselves and would rather do it more themselves, especially when it comes to babies.
I don't think most judges set out to be sexist, which is what separates me from most others who discuss this issue. Most are open to the idea the father could get custody. But all other things considered, it's hard for mothers to not get at least equal custody, unless she is a bad mom or he's got connections and knows how to manipulate the system, which is a different problem altogether.

