[Myrddin]

Colleges also allow students to rate the professors. Those not handing out good grades get slammed and might not be asked to return.

That cuts both ways. It helps weed out profs who are incompetent or total assholes. The students are paying for the professor's time to get quality instruction. The assholes are easy to spot. They don't get enough students signing up for their classes to justify retention.

[TigerClaws]

It might be bad professors. But it might also be professors expecting college students to perform at a reasonable level of competence.