Another option might be that people are increasingly using LLMs to write the books.

This is it. I know of too many companies (in nonfiction, specifically) trying to offload as much as possible to LLMs, damn the quality. Zero chance I buy any nonfiction written after 2021 or so unless it comes with strong recommendations from sources I trust. No more on-a-whim purchases because something looks interesting and is on sale or whatever. That’s over.