I not infrequently find myself prefixing my own code review comments with some variant of "I'm mostly asking this to make myself sound smart, but..." and then usually (not always) I delete the comment. But it's not a high proportion of my total review output, assuming I'm impartial enough to tell the difference.

My point is, it definitely happens, but if it applies to _most_ PR comments I think the code review practices of your project may need improving. My experience has been that most code review comments are either seeking to impart or gain knowledge that the commenter genuinely finds useful.