You're misunderstanding the lawsuit.
The lawsuit isn't about the 30% that Apple charges for purchase of applications in the app store. Epic isn't suing Apple because Apple charges them 30% for the sale of Fortnite.
The lawsuit is about the in app transactions. Epic sells cosmetics in fortnite, using Epic's payment processor, in Epic's application, with downloads coming from Epic's servers to Epic's clients, in Epic's application. Apple is demanding a cut of that. Epic is suing them cause that's a bit silly.
A brick and mortar comparison would be this:
You walk into Bestbuy/Staples/Target/Walmart/etc... take your pick, you buy a Dell Laptop and take that laptop home. You pay Dell X dollars, and X-30% goes to dell, 30% goes to the Brick and Mortar store of your choice (B&M). All's good so far, this makes sense, Dell pays B&M to have their laptops on display, B&M puts them on display, and you buy them, everyone gets what they want.
Now, you go home with that laptop, and decide that you want a keyboard, mouse and monitor to go with it as well. So you open up your Dell laptop, connect to your wifi, and go to dell.com. You find a nice Dell Keyboard, a sweet Dell mouse, and a fantastic 4k Dell Monitor to go with your shiny new Dell laptop. You check out on Dell's website, you use your credit card on Dell's Payment processor, and you finalize your Dell purchase with an email from Dell's email servers. Dell then packages the nice fancy new computer equipment into a Dell Box, in a Dell warehouse, and contact a Dell delivery employee who drives a Dell truck out to your house to drop off your new Dell hardware at your house.
At no point in this transaction now has B&M been involved. And in the real world, that's the end of it.
But in Apple's world, B&M now contacts Dell, and mentions that that Dell laptop that this equipment is for was purchased through them, so Dell owe's B&M 30% of the purchase of the Dell Mouse, Dell Keyboard, and Dell monitor that were all purchased from Dell directly, because the original sale was theirs.
This is why Epic is suing Apple.