I think it's probably has something to do with the fact that females have all the biological equipment necessary for child-rearing and males have all the biological equipment necessary to hit it and quit it. Furthermore, only the female can be absolutely certain (without some investigation) that her child is actually
her child. The male usually has to accept some deception risk.
The instinct for pair-bonding--and, in humans, the cultural institution of marriage--evolved as behavioral adaptations to the unfairnesses inherent in having two sexes.
It would appear that equal outcomes demand equal commitment. As the biological commitment is a few kcal and a few minutes for one, versus years of time and many more kcal for the other, that leaves an awful lot of effort to make those scales balance.
Simple analysis suggests that females should cartelize and police each other's behavior with respect to males, embargoing reproduction from any male deemed too selfish in regard to child-rearing responsibilities. But cartels are tough to keep together, and males can counter with misogynistic and patriarchal strategies of their own.
The biological discrepancy will therefore likely never be resolved to anything even close to fairness.
It's never going to be free, either. There is always an opportunity cost. Providing resources to an existing child precludes using them yourself or applying them to a theoretical future child.