I have issues with your interpretations of both papers. High level: The paper on authors says it's Amazons monopoly rent seeking as why writers don't get paid more Music is also famously an industry where artists rarely own the copyright for their (recorded) music, instead selling it to cover production and advertising costs. Also, the music study suffers from and the paper on musicians has a good deal of classical performers and teachers in the mix skewing the sample. These are not professionals that interact with copyright in any meaningful sense, but they are also not professionals that would be classified as creators.

Of course, because music can be performed. People will pay to see a live performance, which means music performers aren't entirely reliant on selling recorded performances. Live readings, on the other hand, don't attract large crowds and therefore aren't a great source of income, making writers more dependent on IP rights. It's pretty obvious.