The reason so many musicians are still on Spotify is because about 30% of all streams come from Spotify, with all the other streaming services trailing far behind. Spotify is still the number one place a lot of artists get discovered, and see the largest amount of revenue just from sheer volume, rather than per stream.
https://explodingtopics.com/blog/music-streaming-stats
The reason most listeners are on Spotify is b/c they have first in market brand recognition as a standalone streaming service.
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