I’m referring to songs that the other apps also have. As an example, both Spotify and TIDAL have licenses for - and will happily play - the vast majority of the North American catalog. Spotify has more user-created public playlists, but not a ton more music.

I mean collections of songs (ie playlists) are themselves copyrightable. Even if they have the songs licenced, they cannot copy playlists from Spotify without risking legal action.

See: https://en.m.wikipedia.org/wiki/Copyright_in_compilation

Spotify actually got sued after some users uploaded playlists that copied ministry of sound playlists, so there is some history of actual litigation in this area.

Thanks, that's fascinating. At least from a cursory search (https://www.google.com/search?q=are+public+playlists+copyrig...), conclusive case law doesn't seem to exist in the US. In the UK, the one case I found was settled (https://www.theguardian.com/technology/2014/feb/27/spotify-m...) and didn't address an app/company attempting to enforce a copyright license received when a user created a playlist. It's not clear in either jurisdiction, but like you said, litigation is definitely possible.