> I would think the OS, the hardware, the payments, the reviews, the App Store, etc.?

But it's not like Epic is selling that, or asking Apple to allow them to have their own manufacturing plant for iPhones, is it?

The iPhone is an instruction-executing machine. That's what people buy. Epic wants to offer a specific set of instructions that people who already use the iPhone can execute. Why should Epic have to go through Apple to do it, if there's nothing in that machine that would prevent Epic from doing it on their own, besides artificial limitations which only have that specific purpose?