That may be true for bigger artists on major labels, but for smaller independent bands it’s not always the case. I am a heavy user and fan of Bandcamp for listening to and purchasing music but I use Spotify for listening in the car and sharing playlists. I often find albums that are only available on either Spotify or Bandcamp but not both.

The ones that aren’t available on Spotify tend to be self-released but otherwise there isn’t much of a pattern. Albums not on Bandcamp, though, tend to be mediocre at best.

And that’s not even mentioning bands that are pulling their music from Spotify in protest…

That's unfortunately not true.

In the US, this song is unavailable on Spotify where I found it, but available on YT music. Preface by Man Without Country. Given another 5 minutes, I could also find a song that is not listed on one but available on another.

https://music.you tube.com/watch?v=bvWjybBBFYs