To have the same profit, Spotify has to charge $13/mo when apple music charges $10/mo with all else being the same.
That's very obviously the App Store monopoly being used to give Apple Music a massive unfair advantage that is practically impossible to break through.
Steam does not have anything like that, if someone else decides to make "Epic Game Launcher" tomorrow for PC, that new company doesn't need to distribute it on the "Steam App Store" and pay valve 30% of all sales.