In all species, adults have a preference for their own biological offspring. Humans are no different.

The love adults have for one another, in the sense of a lover of spouse, is quite different from the love of a child.

Incest would increase the chance of certain defects in a person's children. Which is why people tend to shy away from it. And which is also why that mutts are healthier than pure bred dogs(which are often highly inbred)

Survival of the fittest means that those best adapted to their environment pass their genes on to the next generation. Away of doing that is taking care of your own biological kids.

Why waste resources and time on kids that are not your own?

Most people in the world have and raise their own children. It's the norm to want one's own biological kids.

As far as 30th cousin goes, well, people are closer to their own children than someone else's kids.