Why are so many questions in the Etiquette category related to other people's manners rather than their own?

Because I think human beings are more inclined to see flaws in others first, rather than themselves. 

Why are so many questions in the Etiquette category not related to etiquette AT ALL, but to "Should I dump my abusive boyfriend?" and "My teacher hates me" ?

The point of such questions is to learn how respond to others' behavior, not to control it.

cause noone wants to admit they have bad manners

People want their feelings validated. “I’m offended by that person’s behavior. Please agree with me that they’re rude so I know I’m not being unreasonable.”