Spotify is a service whereas GP is an application platform. You distribute apps (services like Spotify) through GP, whereas you distribute product (songs) through Spotify. GP is the only (reasonable) way to get apps on Android, whereas there are dozens of legal licensed alternatives for Spotify.

You can try to find alternatives for an app store, but that doesn't change the fact that inherently within Android, there are certain roadblocks that simply don't exist for a Spotify competitor, for example.