Not sure licensing is the issue as much as it is just making their business plan more complicated. Google and Apple solve the user files issue because they also sell cloud storage. Your local files count against your iCloud/Google Drive storage cap.
Whatever the reason, I really wish Spotify would work this into their service offering. I have some remixes, and music from friends who DJ part time, sort of like a hobby. It would be nice to access my playlists from the Spotify browser app.