It's because that we expect that anybody can create new artistic works, so if a particular song you want is under copyright, then you can always create a new one.
On the other hand, patents can be critical to industrial and technological progress, and nobody would tell you to simply make your own transistor to work around the original patent.
When artistic copyrights start severely holding society back, then this will change. Until then, when was the last time society was severely hurt by a work being under copyright?
This isn't an issue of entitlement because nobody will ever agree who's entitled to earn what.