Because the like teaching and believe in giving their students/customers the best possible education?

I get not wanting to waste the time of the better students, but if too many student are behind, whose time are you really wasting?

But it goes both ways. If a student doesn't have the prerequisite knowledge for a class it is absolutely unfair and decidedly not the best possible education to slow the class down for students who are prepared. If a class requires X, and you don't have X, that's a you problem, not a university/teacher problem.