Tribalism. As resources diminish, we have an instinctive need to defend what we need to keep ourselves and our ‘tribe’ supplied. Driving out ‘others’ means more for ‘us’.

It’s just that some people have a smaller mental group of ‘us’ than others. You see all people in America as ‘us’. They do not. (Unless it’s an ‘other’ they personally know; then they get included in their mental ‘us’ as ‘one of the good ones’.)

Of course there’s also the fact that much of the scarcity is artificially imposed by the uber-rich to get people into this easily exploitable mindset but that’s a separate discussion.