The problem is that music isn’t generally FOSS - people would like to be paid for their work. So the next best thing is to set up your own music service where you can add your own music.
Here’s a good discussion: https://www.reddit.com/r/selfhosted/comments/rdxbxp/is_there_any_spotifylike_selfhosted_music/
That’s true. However, FOSS usually means that it’s free (F) and open-source (OS). Music isn’t usually open source - they don’t usually share the sheet music or allow free editing/covering/forking of the music. So the worlds don’t completely mesh. You can look at a FOSS solution for managing music, but you won’t find a ton of FOSS music.