Because someone has to raise the kids, and mothers _usually_ but not always take on that role voluntarily. Especially since infants are biologically tied to their mother for food.

My wife is more educated than I, with a masters degree. But she wanted to stay home, work part time, and not have our kids raised by a third party.

The traditional family structure for Homo sapiens has strong biological and environmental roots, which in turn caused "societal traditions". Changing that in a few generations seems quite unlikely.