After a certain point in a career you realize that time spent coddling and being coddled is simply wasted. As a teacher you realize that even though you can be (and likely
are wrong), your job is to stand firm and be direct. If you have no concrete reason to doubt yourself, why be equivocal with someone? And if you do have substantial doubts, why would you risk poisoning the well? In my experience, good professors are both more direct but also quick to shut up because there's no good response--either because a criticism is legitimate or because the student is just being argumentative, but either way there's not much else to say that the student shouldn't be capable of figuring out themselves.
As a student you realize that instructors aren't infallible and that you're not only free but expected to accept or reject what you're told on its merits. And, frankly, this is much easier to do when an instructor isn't equivocal--when they state plainly and with conviction their beliefs. If everybody is mature enough, it's implicitly understood that people can be and are wrong. Unfortunately, not everybody is mature enough.
This is true of leadership more generally. The job of a leader is to put themselves on the line. By being direct and forceful there's never any doubt about where responsibility for an idea lies, and there's no back peddling if they didn't equivocate. It's more difficult to say that an idea wasn't given a proper defense if the defender literally puts it all on the line; where there's no reason to doubt they didn't explore every reasonable criticism.
It's the same thing with writing. As an undergraduate I hated writing papers because the thought of committing to paper an idea I was unsure of seemed wrong. But that's not the point of writing; the point is to commit to paper the best defense you can think of at that very moment, and to do so without equivocation. The criticisms can and will come, as they should. If you equivocate, however, you're softening the blows of subsequent criticisms, which is a disservice to everybody, but especially yourself. Making yourself vulnerable to criticism is a very difficult thing to do.
Especially once you get into graduate and professional studies, the professor's job is present their ideas firmly. To do anything less is a disservice to students and the truth. This is easier for some people than others. People who are legitimately unsympathetic a-holes will, of course, have an easier time at being firm. You can usually differentiate the non a-holes as they're more likely to invest more time in thoughtful, personalized criticism (e.g. 1 1/2 pages of comments). But that takes tremendous effort and patience, and even the non a-holes don't always have the time or ability to do that. The best gift they can give you is serving themselves up as a punching bag.