The problem is that listening habits are different than viewing habits. People generally don't rewatch TV shows (with a few notable exceptions), whereas people regularly re-listen to music. This means that a back catalog is very important in music, but not as important in TV. And since the music back catalog is owned by a collection of different record labels, music services are beholden to those labels. You can create all the new music you want, but if you don't have something approaching 100% of the music from the past, then you're going to be non-viable as a product offering.