Because they're bigots. They're only nice to their own people.
You act like that's weird, or even not totally realistic.
The real world is full of people who are like "I love hanging out with my own people!
But those other people...they're scum, and the world would be better off without them..."
It's like the entire basis for bigotry. And bigotry is the core of the villain's motivation in JJK. "Humans are vermin that deserve to be exterminated."