Because the composer/artist has to be dead for many years for the copyright to expire (in the United states 70 years after the death; cf. https://en.wikipedia.org/w/index.php?title=List_of_countries...).

It isn't, forgive my clumsy writing.

I think I wrote that because classical artists think in terms of performances of a score, and I perceive that pop/rock musicians think in terms of tunes that they have composed.