Actually, there are many species who share the burden of child rearing equally. It is often a matter of resource availability, if food is scarce, males become aggressive and patriarchal structures emerge more prominently, and vice versa. Human history has been harsh, so it is not surprising that our ancestors tended to that type of society.

So, it is simply not true “across all” anything. I will forego badly parroting what actual experts say, so instead I recommend Robert Sapolsky’s lectures on this in his biology courses at Stanford. They’re available on YouTube.

I also notice you just vaguely referred to various animals in instead of humans, which is curious.

My position here, to be clear, is that there are gender differences in personality and phenotype, but they are mostly inconsequential for child rearing in a resource-rich society, ie, both genders are equally competent parents. Individual variation outweighs gender differences.