Exactly, game developers publish on Steam because they want to be on Steam while the App Store is the only way to publish for iOS and the Play Store is also heavily favored by the Android OS to make other avenues for publishing apps unrealistic. That said, a large value of Steam is the network effect (users wanting to have everything in the same store/launcher, friends on Steam for multiplayer) rather than platform features that others could directly compete with so I don't think the cut Valve takes is beyond criticism.