Maybe because everything LLM-written is written in the same style with no creativity, diversity, or idiosyncrasies? If all humans suddenly started writing in a single, bland, corporate style, that would be a tragedy, LLMs or not.

Because right now humans do have a magical ability machines don't. LLMs are a fuzzy reflection of what they've seen hundreds of times already, they don't have originality or intelligence (yet).

As a much more immediate practical matter, LLMs trained on LLM output makes them worse overall, they degrade from doing that. So the more LLM-prodoced content fills the web, the less useful it is as a data source for future LLM training. In addition to just being increasingly boring and vapid.