The person most biologically suited to care for an infant is the person who birthed the infant in the first place, and there is a path dependence inherent in ongoing childcare.
Once you have spent six months or a year raising an infant, it is personally psychologically difficult (and developmentally questionable) to cut that tie.
I get that it’s ideologically convenient to pretend that men and women are interchangeable economic cogs, but the sexes are fundamentally different in very pertinent ways.
We all lose under this model, but ultimately the people who lose the most when economic and social policy rejects our inconvenient biological differences are the children themselves.
Family should be able to support themselves on a single earner’s salary. That’s what we should prioritize. They can decide themselves who should stay home, if anyone; gender doesn’t need to factor into our policy if we simply make it our goal for families to not require dual incomes.
