story
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..