Humans do remix stuff when we create, and copyright draws a kind of arbitrary line around when that becomes original and when it becomes derivative, mostly because if it didn't then it would be impossible for us to get new books, movies, and songs.
All copyright is a hack; it's not an ideological, internally consistent framework. It is a set of fallible legal rules we invented so that people would get paid for creating things we care about. That doesn't mean that comparisons aren't useful or that we can't extrapolate from the existing rules, but even where fair use is concerned, the actual justification isn't a logical one, it's: "if we didn't have this standard nobody would make anything." So the boundaries around fair use and what things fall into it are not being created purely based on logic or first principles.
The reason why we have copyright is because we want people to be paid for making creative work. The reason we have fair use and a standard of "originality" that treats coders/artists learning from other artists as acceptable is because if we didn't, the entire system would fall apart and nobody would be able to make new creative works.
Everything in copyright exists purely to get people to make more stuff in a sustainable way. It's an outcome-driven process.
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More recently, there are a lot of people who argue that IP is a real, fundamental property right, but frankly, IP doesn't stand up at all if you think about it too hard. The justifications for why IP theft is theft can't be consistently generalized in a way that applies outside of the IP space. The standards for what does and doesn't count as creative aren't really consistent or based on a straightforward definition of creativity.
A lot of people would love to say that IP rights are just property rights, but... it's not all that convincing, and the history of copyright doesn't really indicate to me that the people building the laws thought of them that way.
And again, that doesn't mean that there's no consistency in copyright rules or that copyright rulings don't have implications beyond the original rulings. But it is almost always easier to think about copyright and almost always easier to understand why copyright laws are the way that they are if you approach copyright as a means to an end, and understand the existing laws not as an attempt to create an internally consistent system, but as a series of attempts throughout US history to achieve a consistent publicly beneficial outcome.
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With that in mind, I suspect whether or not AI works count as remixing is largely going to be decided based on commercial interests, individual judges, and individual juries, possibly with input from US legislature.
"Everybody remixes" historically hasn't been the most useful argument during these debates? So I don't know how it's going to play out this time around. I vaguely suspect it's going to come down to whether or not individual pieces are recognizable? That's how we got wild copyright laws about some individual chord progressions being treated as derivative works in songs.