I don’t know. Tradition? Pride, wanting to be sure there is someone you will always be better than? Economics. Needing labor you can starve and overwork to be able to afford the life you want, and needing to believe that you aren’t wrong to do it. I have known men—and women, of course—to mistreat someone first, then hate them second.

[ He’s used it to his advantage, is what he means, on the occasions he needed a person with those inclinations to despise someone. Perhaps to despise him. ]

I suspect because every time they see them they feel some disgust they refuse to look at directly. So it becomes this person disgusts me, rather than I disgust myself.