Historically professors have tenure (admittedly less so now, I believe), so how they score on evaluations has no real bearing on them keeping their jobs.

I had a professor who was a terrible lecturer, and he knew his student evaluations were terrible every time. As he handed out the evaluations he would tell us that he did not read them, and did not care what we wrote.

I graduated twenty years ago, but just checked the department's website. He's still employed!