I think the main difference between derivative/inspired works created by humans and those created by AI is the presence of "creative effort." This is something that humans can do, but narrow AI cannot.
Even bland statements humans make about nonfiction facts have some creativity in them, even if the ideas are non-copyrightable (e.g., I cannot copyright the fact that the declaration of independence was signed in 1776. However, the exact way I present this fact can be copyrightable- a timeline, chart, table, passage of text, etc. could all be copyrightable).
"Creative effort" is a hard thing to pin down, since "effort" alone does not qualify (e.g., I can't copyright a phone directory even if I spent a lot of effort collecting names/numbers, since simply putting names and numbers alongside each other in alphabetical isn't particularly creative or original). I don't think there's really a bright line test for what constitutes as "creative," but it doesn't take a lot. Randomness doesn't qualify either (e.g., I can't just pick a random stone out of a stream and declare copyright on it, even if it's a very unique-looking rock).
Narrow AI is ultimately just a very complex algorithm created based on training data. This is oversimplifying a lot of steps involved, but there isn't anything "creative" or "subjective" involved in how an LLM creates passages of text. At most, I think you could say that the developers of the AI have copyright over the initial code used to make that AI. I think that the outputs of some functional AI could be copyrightable by its developers, but I don't think any machine-learning AI would really qualify if it's the sole source of the work.
Personally, I think that the results of what an AI like Midjourney or ChatGPT creates would fall under public domain. Most of the time, it's removed enough from the source material that it's not really derivative anymore. However, I think if someone were to prompt one of these AI to create a work that explicitly mimics that of an author or artist, that could be infringement.
IANAL, this is just one random internet user's opinion.