Because it's not socially acceptable to take a hiatus _instead_ of the mom and not economically viable for both parents to take a few months (or ideally a year

Because it's not socially acceptable to take a hiatus _instead_ of the mom and not economically viable for both parents to take a few months (or ideally a year or more) to take care of the newborn. Because men can't breastfeed. Because mothers are more attached to their children, instinctively and psychologically. There are dozens of reasons "why".