Because on the job market you compete against your neighbors. You can't do it now, because you can't get the neighbors from the 50s.
When most of your reference class have one income per family, the labor market will adapt so that you can feed the family from one income, because your neighbors will refuse to work for less. Then also you can get an income sufficient to feed your family.
When most of your reference class have two incomes per family, the labor market adapts so that now you need two incomes to feed your family. Your neighbors are willing to work for half the cost, and to remain competitive so do you.
(Here, "feed your family" is an euphemism for... basically the total cost of living on the same standard as your neighbors. That includes literally food, but also the cost of housing, schools for kids, health care, etc. A part of it you can save by living more cheaply than your neighbors, for example always cooking at home and never going to restaurants. But you can't do much with the housing costs, and that is a large part of the family budget.)
> Why not just let mothers be mothers?
https://en.wikipedia.org/wiki/Prisoner%27s_dilemma -- if all mothers stay at home, they can. If half of them chooses career, the other half has to.
For those who supported the government, it was much more expensive.
For those who were persecuted by the government, it was a living nightmare.
With regards to productivity and pay, I share with you many illustrative charts. https://wtfhappenedin1971.com/
We are hearing "automation will destroy jobs" left and right and for some reason, we can't afford stay at home moms.
The "universal child care" argument makes me feel uncomfortable. It one step short of a dystopia. One where women would surrender their child to a state sponsored center where child care specialists make them productive members of society. Freed from their child rearing duty, "mothers" can continue working efficiently.
Why not just let mothers be mothers? What's the point of progress if we can't even do that. And also turn gender equality the right way, by letting fathers be fathers instead of requiring both parents to work full time.