I think it's mostly socialisation.
I rejected femininity fairly young (I worry that girls just like me are probably expected to trans now, but that's another topic). So I missed out on or rebelled against a lot of the expectations and socialisation. I've never had the desire to care for other humans (though I did work with animals for a long time, they're ok).
There are some innate biological differences, yes, but I think the whole 'I think I wanted to give up work because hormones' is a bit facetious. I'm still breastfeeding my 18 month old and I love her to bits (as does DH), but I was happy to go back to work when she was 11 months! Mat leave was like an extended holiday from life where it was easier to keep on top of the laundry but quite mind-numbing.
The extra hormones have not left me with any additional desire to care for any other humans.
I think you either have it or you don't, but boys are more likely to discouraged from caring roles by societal expectations, and women are more likely to do them out of a lack of choice, because they fit around the family (because we're still the 'main carer' in most cases).
I think the socialisation from birth is even worse now if anything, and it's one of the reasons DD is often dressed 'like a boy' because I'm hoping that people will see a dinosaur hoodie and assume she can do whatever she wants, because I know that most people unconsciously think 'fragile' when they see pink dresses. Obviously when she's old enough to choose she can choose, but we will be making sure she knows she can still do anything she wants.
Ditto if we'd had a boy he'd have had all the choices in the world (I wouldn't have dressed him in pink dresses to prove a point, because I think it's the socialisation of little girls that's the problem at this age, I don't think the issues around the socialisation of boys come in until a bit later).