Why can’t I sell skins in Fortnite?

I will start that I am a fan of app stores. I think app stores are our best hope of preventing viruses in normal users. I have loved steam, PlayStation, switch app stores. Windows and mac AppStore’s I admit it tend to skip. I haven’t followed the Apple vs epic lawsuit closely. But as my son has gotten into fortnite we have been spending more money to buy virtual goods on Epic’s fortnite. I have to pay what epic wants. And they don’t even let 3rd parties sell anything. I don’t understand why they can tell say to Apple if I sell on your platform, it isn’t fair to say you can do it but I get 30%. If the person who is selling gets to set the amount – won’t that kill all platforms like apple, PlayStation, x-box because all developers in the end would want to pay nothing and keep all the profits. I don’t get how Apple is monopolistic. Yes, Apple controls all iPhones but with something like 50% of US market, and 15% of world market of all cell phones that doesn’t sound like a monopoly to me. Sure a monopoly in iPhones. But then Sony is a monopoly in PS4 and Microsoft a monopoly in Xbox. And epic is a monopoly in fortnite. Destroying App Stores in iOS and within games like fortnite will in the end hurt developers and costumers. Companies have the right to do with their hardware and software what they want – as long as they aren’t true monopolies – which clearly neither Apple nor Epic are. I don’t know if 30% is fair – it does seem quite high to me – but let the market decide that not the courts.