Just like the application developers on Google Play, Apple's app store, and Steam, eh?
The only "pull factor" Spotify has is the userbase, which is arguably fair. For Google, though, the "pull factor" is because you literally cannot sell directly to users through the Play Store.
One could argue (on Android) that it's possible that you can install third-party markets, and that's true, but there's also pretty big roadblocks to installing, whereas selling on another music platform is much easier in comparison. (Don't believe me? Send your father an APK file and ask him to install it. Let alone iOS where this is not possible at all..
Couldn't you replace "Google" and "Play Store" with "Spotify"?
You can "sell" your app directly to users through your own site (see Fortnite) or dubious sites like https://en.uptodown.com/android/general-android but you have the same "pull factor" with the Play Store that you do with Spotify (the user base).
The Google + Play Store and Spotify (company) + Spotify (platforms) situations don't seem fundamentally different to me, just different by degrees. Google's monopoly on Android app distribution is unparalleled.
You can try to find alternatives for an app store, but that doesn't change the fact that inherently within Android, there are certain roadblocks that simply don't exist for a Spotify competitor, for example.
Spotify is just a convenience service in the end. They could sell their music through their own app, site, Magnatune, etc.
Even Google Play is iffy, as developers could distribute through 3rd party app stores or sideloading, though Google makes that harder than it should be.
Apple is really the only one of the 3 you mentioned that forces a 15/30% cut if developers want to distribute an app on their platform.