To stop the same two companies from owning everything? YouTube Music and Apple Music are shameless anticompetitive moves, leveraging market dominance to move into other existing markets. (I'll afford more lenience to Apple Music, since iTunes was already huge, being the undisputed king of music sales before streaming subscriptions took off.)
I've also been using Spotify for longer than YouTube Music, or its predecessor that Google killed (as they do periodically) even exited.
It's quite funny to me to frame Spotify as an underdog, though I suppose there's truth to that, because of the sheer size of Apple and Google. I've never thought of it that way.
I mostly listen to albums, the Spotify “library” is a reasonable way to browse my saved albums.
Spotify (now) supports lossless.
Spotify connects to Sonos, wiim, etc. devices
Spotify supports marking albums and playlists for offline sync, including to my Garmin watch.
I participate in a number of collaborative Spotify playlists (e.g. on group trips, at parties, etc.). I’ve never seen anyone make a collaborative playlist on another platform, much less missed out on participating in one.
Shazam results have an “Open in Spotify” button and Shazam adds everything it identifies to a Spotify playlist.
When I’ve used it, the YouTube Music UI has felt like it’s not really designed for people who listen to music the way I do at all.
I’m not willing to go without YouTube just to spite Google but I’d rather not give them money or attention/usage if I can avoid it.
I don’t know how many of these would also be ok with YouTube Music, but it’s clearly not all of them and I suspect it’s close to zero. I’m fortunate that the cost of Spotify is not a burden for me, and I’d much rather pay it to get closer to the experience I want than try to get by with YouTube Music.
Pro Spotify: existing playlists and history, better artists info, better UI.
YouTube Music is both better and worse: UI has some usability issues and unfortunately it shares likes and playlists with the normal YouTube account, as a library it has lots of crap uploaded by YouTube users, often wrong metadata, but thanks to that it also has some niche artists and recordings which are not available on other platforms.
But, doesn't Youtube Premium include Youtube Music? So why pay for Spotify premium too?