You do realize that is exactly what Epic was trying to do? Fortnite is the app. Apple controls the distribution of it. In app purchases have NOTHING to do with Apple. Epic wants to charge for those, since Apple DOES NOTHING to get the money for those! How is that so hard to understand?
Since you're familiar with Walmart, does Walmart get a piece of every streaming service you purchase for the TV you bought from Walmart? Does HBO, Netflix, Hulu, etc., give Walmart 30% of what you pay streaming services because you got your TV from Walmart? Of course not, so why does Apple feel they should deserve such huge amounts of money for something they have no hand in (or at least they wouldn't if Epic could process their own payments).
I'm so tired of analogies that aren't even proper! Once the app has been downloaded to your device, Apple's job is done (or it should be, were it not for the fact that Apple insists on processing the payment for items in the app Apple doesn't own or control). They shouldn't get a dime for IAP's. Maybe there shouldn't be free apps if there are IAP?