While yes a lot of music is based on prior art there are levels of that and there is a lot of case law on what doesn’t break copyright. There is a lot of music that steals from classical but that is public domain. There are rules about how much of a riff from a copyrighted song you can borrow before it becomes copyright infringement and you owe the original writer royalties. Music also has a lot of standard conventions that lead to similar conclusions at least if you are using western music theory since that is based on western classical music.

So an amount of this deals with scale and the copyright status of the prior art.