As for Spotify et al, label artists can't opt out. Their contracts won't let them. The reason Taylor Swift's current album isn't on any streaming services is because she makes a ton more selling it and streaming would cannibalize sales. And her current album (and only her current album) she did without a major label... so she's one of the few that gets to make that call.
Streaming services want current music to get and keep subscribers. Musicians want their latest music to be sales and their back catalog streaming (ala Netflix, etc) to maximize income.