People get a lot of gratification in raising children. Something that people without children can't really understand fully. I know I didn't understand it until I had children. You're building (for lack of a better word) something that will outlast you, the original legacy, one that nearly anyone can achieve. If you spend time helping people in need, I mean really spending time and energy, not just donating or virtue signaling, it's similar. An example would be teaching or coaching; being a big brother or adopting a child; being a good aunt or uncle; mentoring even, to a lesser degree, etc. Taking care of and benefiting someone who depends on you doing it really gives a lot of gratification.
>reduces humanity back down to level of any other simpler life form. Eat, fuck, die, repeat.
It's a little more than that but though humans (most) are civilized and intelligent, we're still a great ape.
>It's a belief I find hugely depressing.
Why? It's our nature. Jonas Salk is probably the finest human I can think of and he was certainly motivated by all those things.