Maybe the artists think consumers should pay more. If Spotify wasn't a quasi-monopoly, it would seem silly for artists to demand consumers pay more only on the Spotify platform, but since Spotify is, the argument is at least prima facie reasonable?
https://www.statista.com/statistics/653926/music-streaming-s...
I'd argue that all music-streaming services have very little pricing power!
Streaming services rely on being an easier/friction-free option as opposed to piracy (or alternatively, buying the copies directly from the musicians).
Your point is still valid though, the competition is still fierce; Amazon, Tidal, Apple.. etc.