How are most humans egoists?
It is a matter of evolution.
Humans and all other animals are egoists.
But you could say that it is a kind of defense. Caring more about yourself than others ensures your survival in most cases.
It is something that we got in our evolution. Something we still need today.
Like back in the old days, a few thousand years ago. If you were being hunted by a predator together with another human that you didn't know, you would rather that he/she got caught than yourself, to ensure YOUR survival.
To ensure that you survive to pass on your genes. Because that is what life is all about, and what evolution has adapted us to, to pass on our genes.
And egoists are often very self-secure, which makes them more attractive.
Unless they are too egoistic of course.