Downloading is a one-time transaction. After the app is downloaded on the user's device, Apple doesn't provide much if any service to the app's users (save for the instant notifications maybe, but that is minuscule).
It's EPIC's game servers that do the heavy lifting server-wise and provide the actual gaming connectivity. So why should Apple take a cut of that, if they don't provide the actual gaming online server/service?
Where were the complaints when Sony, Nintendo and Microsoft took 60% to 70% of every cartridge or disc sold and required developers to pay up-front money to license the SDK's to develop?
Apple made this easy for ANYBODY to make a game and took the commissions and cut them in half (or much less for subscription-based after the first year).
When every game has its own payment system it is:
A) a pain for the user to keep entering their credit card
B) a pain for the user to deal with "what happens if one of 100 developer's credit card databases gets hacked"
C) a pain for Apple to sift through customer support calls when the customer does not know who is charging them by mistake -- especially when their credit card gets hacked
D) a pain for the user to try to manage parental controls to block in-app purchases in 100 different ways for different developers
EPIC should shut up and develop their own platform if they think they can do it. Steam did it on Windows and macOS and it is fairly successful, but their are costs in setting that up. Apple makes it possible for anybody to distribute their apps and games to everybody.
And if your app/game is free, Apple just eats the cost of hosting it, reviewing it and managing the upgrade system to replace the binaries when you release a new version -- that alone is awesome since writing installers and especially upgrade installers and automatic update checking and notification is a total pain in the butt and Apple handles a bunch of that for the developer.
When Nintendo and Sony and Microsoft Xbox and even Atari built their platforms nobody ever questioned whether or not they were entitled to the huge fees they charged. This is 100% spin by EPIC games. Screw Fortnite.