I know this heavily depends on the location, but at our university most professors weren't there to teach students. Giving lectures was more like the price of entry, something that has to be done to keep the machine alive. Their actual job and passion was doing research, publishing highly-cited papers, going to conferences, and furthering their and by extension their university's reputation.

It might be different if being a grad student was about giving awesome lectures, but usually it's about doing awesome research; and those that are best at doing and publishing research then get tenure-track.