Selling virtual items is not especially common due to the exposure it creates to fraud and abuse. Fortnite doesn't allow selling or trading - games that do like Team Fortress 2 and Counter-strike Go are home to lots of scammers who try to trick teens and other uninformed players into giving away their valuable loot :(
Who are you going to buy from without giving Epic a cut? They're the only ones offering Fortnite items.
Apple doesn't just require you use their payment processor - which might in theory be acceptable - they also demand 30 cents on every dollar. Payment providers everywhere have shown that fraud and abuse can be mitigated to an acceptable level with transaction fees as low as half a cent on the dollar. So Apple forces you into an agreement to use a payment processor that is 30-60x more expensive than the competition simply because they can.
Correct for credit card transactions but not for gift card credit which A LOT of people use. Far more than you’d assume. Apple doesn’t get all $50 when you buy an Apple gift card. I’d be surprised if they even got net $40 per $50 card.
The physical cards cost money to produce and the retail stores will insist upon a decent cut. And then the cards are often discounted by 10, 15 or even 20%.
Who are you going to buy from without giving Epic a cut? They're the only ones offering Fortnite items.