This reminds me of conversations around plagiarism that come up when working with students: that question of "this other person expressed this idea better than I can, why can't I just use their writing"?

Because I want to know what you think, because putting our thoughts into words and sharing them is an important part of thinking, because we'll lose these skills if we don't use them, because in thinking for yourself you might come up with something interesting that nobody has ever thought before.

Of course, writers are allowed to reference and use other peoples writing: with proper attribution. I don't have a problem with people sharing quality AI generated content when it's labelled as such. The issue is that most people writing AI comments don't do this, which is itself probably the strongest indictment of the practice.