Maybe some people, I don't know, want to teach? As in it's their calling.
Seriously, you could apply this logiccubject. Why should I listen to my math teacher? They're not working for NASA, they're just teaching a high school class. Why should I pay attention to my foreign language instructor? They're not employed by the UN, obviously they're trash.
Why should I get advice from a bunch of SHaters about how to be more attractive and get sex? Because they're so good at both that they frequent SlutHate complaining about their unattractiveness and lack of sex.