Why do people hate lower castes?

I've been seeing many posts about how "SC/STs are leeches and that they don't deserve seats. Blah blah" and it got me thinking, how is this ok on the internet??? Like if I say "I hate black people" I'm banned if I mean it seriously. And I don't know the history of caste system, I don't know what the caste system does so I'd like to know why people hate lower castes.