Globally, the main thing is that married families have transitioned from small farm cooperatives (where the husband works the fields and the wife in the house) to market-based work for both of them. The problem with market based work is that it's usually incompatible with keeping one eye on small children, unlike the household chores of a farm. Also, kids are pure burdens until they can join the market themselves, whereas farm kids could help collect eggs and feed chickens starting pretty young.

In America at least the cost disease of education, housing, and healthcare all make it worse, but the first part is the primary culprit. And I'm not sure there's any fix for it besides long parental leaves and subsidized childcare.