They explicitly prevent other companies from providing this infrastructure, marketplace and platform. Apple don't offer this stuff as a favour to developers, they demand that developers use it and prevent them from using anything else. Apple aren't some Good Samaritan providing a centralised set of services out of the goodness of their hearts, they are forcing companies to pay an exorbitant fee to be able to play in their garden, and they make billions of dollars in profit from this.
Compare Apple with Steam (who also provide the infrastructure, marketplace and platform). Steam don't force developers to use their services. They're still successful, but you can get almost every game on Steam from somewhere else. This is what I'd ideally like Apple to do. It wouldn't make any difference to me, as I haven't owned or developed for Apple devices in many decades, but it would make a huge difference to many developers, and many device owners. I doubt it will actually make that much difference to Apple's profits, but it would make a difference to the rapidly declining good will they have in the developer community, and increasingly in their customers.