Nature and sexual selection have nothing to do with it. A complete lack of societal support for women who want to have children and maintain a career in STEM academia (or similar) is all there is to it.

>> Many fathers would very much prefer to spend time with their families, but can't because they're expected to first and foremost provide for them.

Why do the fathers "have to provide"? Why is it so difficult for a man to stay at home and take care of the kids, after they're born (which he can obviously not really do)? Is that nature, again?