The issue is that LLMs adopt a very particular style that is a mix of being very polished (em-dash, lists-of-three, etc) that is reminiscent of marketing copy, and some quirks picked up from the humans curating the training data somewhere in Africa

If AI was writing like everyone else we wouldn't be talking about this. But instead it writes like a subset of people write, many of them just some of the time as a conscious effort. An effort that now makes what they write look like lower quality