Your choice of companion is part of how you compete.
People generally pick friends based on what their friends do for them and how their reputation (e.g. influence) is, whether or not they actually realize that. So much of this is done subconsciously.
Nobody wants to keep the friend around who is always asking for help… everybody wants the friend who asks for little and is always there for them.
I do agree that the social construct of nuclear families is not natural in any way. You see this with divorce rates quite clearly. It’s quite common to find the rich, powerful, influential and attractive having many sexual partners and many followers. Without things like child support I suspect there would be many MANY more bastards out there than there already are.
Anyway, you are looking at things through an idealist lens. I’m pointing things out in a blunt narcissistic lens. People are shades of gray, not black and white. If you ignore the competition and pretend everyone will be nice you’ll quickly find this doesn’t work when scaled out past a small community. It just doesn’t. It never has. It never will.