[ProblemFactory]

> For whatever reason-- probably tenure-- he was allowed to stay.

I attended a very good university (by various rankings), and even there were a few terrible lecturers.

They are allowed to stay not because of tenure (they could stop lecturing and do research instead) or departmental indifference (the board of studies takes feedback seriously, and sometimes does "fire" lecturers from teaching particular classes). The reason is instead that there is nobody else to replace them.

The great professors teach a class in their specialty which they are most knowledgeable and enthusiastic about. But after that, there are many small undergraduate classes left over, which nobody wants to teach, but someone has to. Given that the other option would be to throw the class out of the syllabus, bad lecturers still stay as teachers.

One of the deeper reasons is that grant funding and academic careers depend only on the number of articles published. Being a good teacher (or teaching at all) is only a "distraction" done out of good-will or enthusiasm for the topic.