Correct if they did this virtually every really big player would opt out and offer a package on their website. See the MS and apple desktop store.
I believe Apple is ready to die on this anti-malware hill.
This isn't just blind fanboyism, either: PC gamers absolutely loathe Epic Games Store and the Microsoft Store, because they want their entire collection of software to live on Steam. Yes, a decade and change ago everyone hated Steam, but now it's the established way of doing things. The more developer-friendly, competitive option is to have 15 different launchers, one for each publisher, with their own payment systems, terms of service, and so on.
You can see this in the reactions Apple users have to the Epic lawsuit, versus their reaction to stories of Apple demanding their cut from developers that don't have it to give. They're way more sympathetic to companies that aren't trying to split their app libraries in half.
(EDIT: I should probably clarify that I, myself, don't think alternative iOS app stores would actually take off without major cross-subsidies from powerful industry players. Google Play has a stranglehold in markets where it's available, even though Android has no technical lock-out like iOS. EGS on Android is a joke and I would never install it. On the other, OTHER hand, though... Android in China, where Google Play is banned, is basically PC digital downloads, pre-Steam.)
If you want to shop in the mall vote with your wallet if there isn't enough demand to convince merchants to situate themselves there then deal with it.
Laws forcing merchants and customers into the mall ought to be themselves illegal.