I dug a little into one of them and it sounds like it was an independent author (they posted about it on reddit) who I guess didn't want to put the same effort into procuring a cover as writing the book. A lot of authors I think don't have a lot of respect for visual arts and kind of see the cover as a forced labor to publish a book. TBH sci-fi book covers with abstract spaceships and rainbow nebulas are one of the easier things for AI to believably churn out.
But I guess I kind of use the effort put into the cover as a way to gauge how much the author and publisher themselves think the work is worth. Even if I could be sure that the books weren't AI generated themselves (I can't) I left thinking, yeah, I'm probably never going to read again, because I have absolutely no metric, however bad any more, for guessing about the quality of a the book.