My pet theory comes to information density. LLM writing feels too shallow. If you read a text from a person who cares about her subject, there will be more useful detail and insight scatterd throughout. And the information within sentences will be more meaningfully connected. On one hand, LLM would know all that, but it does not seem to know how to distill that knowledge into a written text. I read what people write. If I know the text is from an LLM, I skim.