I’m not sure that’s quite as true as you say.
Women certainly invest a vast amount more in a pregnancy carried to term than their sperm donor did. But once the kid is out of the chute, males and females have an equal ability (not to say proclivity) to invest either lots, little, or zero. As well, in almost any sane accounting the investment of that 9 months is utterly swamped by the investment of the subsequent 15-20 years.
IMO a vast percentage of male indifference to post-birth care is enabled by a corresponding female non-indifference to the same.
We don’t actually know that if women on average cared less, men would or would not care more. Dumping it all on her is vastly convenient from the male POV. But would not work, on average, if the average female said “Screw you, and screw that: you fertilized it, you own it, I did my part, I’m outta here!” as soon as the kid saw the light of day.
Sure, 9 months of pregnancy isn’t much compared to a couple of decades of child-rearing, and a woman can forgo that couple of decades by immediately putting the kid up for adoption after birth… but it’s also a heck of a lot more investment than the man needs. In the time that an R-selection-inclined woman can produce one offspring, an R-selection-inclined man can produce hundreds.
Except there is a significant sex differential regarding ability to feed the child during infancy (and in many cultures, well into early childhood).
There have been a lot of societies where male children were transferred to male care and training from about age five or seven onwards, AFAICT. But of course the crucial childrearing tasks of teaching them to look after their basic needs in, e.g., feeding and cleaning themselves had mostly already been taken care of by mothers and other women. And female children invariably (again AFAICT) remained with the women rather than being cared for by fathers.
I wasn’t suggesting that such human societies had ever existed, just that they could in principle exist.
You raise the excellent point that at least before the invention of safe water supplies and packaged infant formula the minimum biologically required female investment doesn’t end at birth, it ends at weaning. Which adds a bunch to the minimum possible burden, thereby making all the less likely that a society which practices “dump all childcare on the men” might emerge in a pre-technological era.