Because those roots are hard wired into us by a billion years of evolution.
We have a way to go, but we have made at least some progress. Maybe in another few hundred years humans won't be so 'Us and Them'. Maybe materialism will be the next thing we work on. Who knows, maybe by the time we reach the next galaxy we'll have it sorted out.
If the most powerful institution in the western world couldn't "fix" this aspect of human nature over millennia, it is sheer arrogance to think we can do better today.
Something like aggressive behavior though, is something that can be selected against.