It's because when "they" know something that others don't, "they" feel special and superior.
Instead of helping and trying to pass on knowledge, like a good person, "they" prefer to try to humiliate and show superiority.
This is a pattern of disturbed people with low self-esteem who can't see beyond their own navel and are incapable of helping those who don't understand, whether due to laziness, because it's really complicated, or because the teaching method isn't ideally applicable to all types of understanding.