Which is kind of weird, considering how many other annoying things professors need to do. Professors do three or more jobs.
Why is this? Part of it is a feedback loop where the best researchers become professors so professorships are associated with the best research so the best researchers become professors.
Hunch: in the research world in the past it was easier to go from not being a professor to being a professor so there was less pressure to get on tenure track asap.