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!
Fair enough, but even the tenured professors were untenured at some point before so they did matter and were not problematic enough to deny tenure. And tenure doesn't really apply the same for adjuncts or some lecturers, who are perhaps even more dependent on student evals. And of course it depends on the seriousness the university takes teaching, as there are R1s that are more research oriented and care less about teaching, but they account for 1% of all universities.
Even as a grad student, my teaching evaluations couldn't negatively affect me. The only people it could have real, negative consequences for were instructors, who work on contracts. As an example, my department head (perhaps unintentionally) sent everyone an email saying any scores of less than 3.73 on a 1-7 scale were grounds for termination of that instructor's contract. I did not envy them at all, basically being highly educated customer service workers.
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!