Say I'm a publisher; I can sell my games for 5% cheaper than all the competing platforms and let my installer/store languish. All it needs to do is let people buy the game, and my competitors can do the hard work of actually distributing the game, updating it, supporting features like community pages, friend lists, steam workshop etc.
People buy the game from me because it's cheaper, and the other platforms can pay the actual cost of supporting that license.