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It’s an App Store not the UNO, Apple is defending just profit not anything of real worth or value.
please consider this in your evaluation and statements!
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Let's look at the value that Apple and Google provide since you say it is "not anything of real worth of value" (because you are wrong):
1) Apple and Google develop, support and maintain SDK's for developers to build their apps. That includes documentation, bug fixing, etc.... It is far far less expensive to provide SDK's to yourself than to open those up to third parties.
2) Apple and Google both provide trusted payment systems minimizing the places your credit card goes AND they both implement parental controls that parents can trust to prevent unauthorized in-app purchases by their children when playing Fortnite.
3) Apple and Google both promote and distribute the apps on platforms that they built with millions of users worldwide -- all the developer has to do is write the game and publish to the App Store or Play Store and they instantly have exposure to a gigantic customer base with ZERO distribution costs so any developer can participate for a measly $100 annual fee and 15% to 30% per transaction assuming your app generates revenue (free apps are just a cost to Apple and Google)
4) Apple and Google (to a lesser degree) both offer customer support for people to call when there are unexpected are errant charges from app purchases (they refunded me $50 in virtual currency for Rayman when I failed to realize that Apple TV parental controls were locked down). Imagine if there were 10 third-party app stores on each platforms, all those people would still call Apple first seeking help and Apple has to staff those phone lines.
Most of all, if Epic really was capable of distributing their game on their own, then why did their Epic App Store for Android fail to ever take off? One would think that loyal Fortnite players would have downloaded it and used it to save tons of money on purchases of virtual currency. It is because Epic failed to promote and failed to win the trust of the user base. They get those two value-adds from Apple and Google's marketplaces.
Finally, if Epic really thought there is no cost or value in opening up a platform to third parties, then how about this....
1) Epic develops, supports and maintains SDK's for *third parties* to create in-game items, mods and virtual currencies for Fortnite
2) Epic creates a framework for third parties to sell their in-game items, mods and virtual currencies within Fortnite
3) Epic allows third parties to use their own payment processing for their in-game items, mods and virtual currencies
Are you scratching your head yet why Epic does not do that? They would be incurring a huge cost while letting somebody else get a free ride on their user base, loyalty and trust that they had built with their own investment in marketing, development and distribution. Just like Fortnite has value to Epic Games, the iOS and Android ecosystems have value to Apple and Google, respectively. Epic does not deserve a free ride either.
PS: Game producers were paying 60% or more of their wholesale price to console makers before the iOS App Store model came along and that does not even count the 100% markup to retail price that brick-and-mortar stores charged. Game producers went from making 40% or less to 70% or more because of Apple. Game console makers now only charge 30% to developers and YET, Epic has not gone after Sony, Microsoft or Nintendo.
Entering a dev contract with Apple is not as big a deal as some are trying to make it in this thread. In the real world, contracts are voided, renegotiated, or sold on a regular basis; especially in sports. As for real world consequences, what exactly has any of the ISP's, cellular carriers, government contractors, heck I could go on forever, suffered as a result of breaching multiple contracts? 
