I get that, but what I don't get is why the mothers just accept that they have to be the primary carer, if that's not what they want.
To be fair to mothers @musixa , I don't think it's a matter of a completely free choice. Women are conditioned &socialised from birth to be the caring ones, the ones who derive identity & value from being the relational people, iyswim.
And men are conditioned from birth to think that the world is organised around them, and their needs will automatically be met.
Most men would never dream of taking the hit to their self-identity, their fredom, their career, their earning capacity, their physical & mental health, that women routinely take on by parenting.
So I don't think we can throw up our hands and say "Well, it was her choice." I don't think it's ever a free choice for women - probably one of the reasons I didn't push to have a child on my own (cf another thread) when it was clear no man wanted to partner & raise a family with me.