Because that's the only way you can do it.
They can always create their own mobile platform, if they think Apple providing them with one is not worth much to them...
After all Google and Apple did.
Just as Apple could allow them to build their own payment stack into it and circumvent the app store.
But they don't.
Well, you build the environment, you make the rules is kind of the whole point.
Yes, but on mobile Spotify doesn't even provide the Marketplace -- only part of it (their own app), the other (the mobile OS, platform, payment processing, consumer trust, marketing, etc) is built by Apple and Google.