But they are marketing your app by marketing the platform. Those commercials you see with catchy tunes and hands swiping and flicking apps, those make people want an iOS device to run apps. Everything Apple does is to make consumers want to come to the platform, and based on their numbers, it is working. Then you, the developer, gets to put you app in the only marketplace available to those 100M+ users. That is the value Apple is bringing.

It is the same value other market places bring. People sell on eBay because they know there are eyeballs there. Same with people who sell on Amazon. eBay and Amazon market their platforms to bring that value. No one creates their own webpage when they want to sell some stuff and hope people find it through Google. They go to where the hard part, bringing people to look, has already been done.