I challenge you to listen to 4 chords of awesome and tell me again about how every song is completely original. How does eragon exist when it's definitely ripped parts from star wars, etc...ai usually doesn't spit out a full plagiarism, but a loosely inspired work which is what most media we consume is.
A copyright in just Eragon would be incredibly thin, for the exact reasons you state. This criticism of copyright by people that have no understanding of actual copyright law, how it works, how its used, etc, is so exhausting and ignorant.
Edit: 4 chords of awesome link is https://youtube.com/watch?v=oOlDewpCfZQ&si=8vL6PbDnHiaffJh3