What great features do Android users get as a result of being able to sideload apps which aren’t available on the App Store?
I believe the (weak) answer to the above is the answer to your question. Features are what matters to the majority of users. The ability to sideload apps itself is not.
So let's say you buy a game for $10 and the creator gets only $7 (as Apple keeps $3). But there could be an alterative app store, where the same game would be sold for $9, and the creator would get $8 (the store would keep $1). So the reason is, that with competition, the price of software could decrease and you would save money.
I don't mind that Apple keeps a large commission. But I mind the fact, that Apple has a monopoly on iOS software distribution, and the monopoly is always bad for the customers.
I believe the (weak) answer to the above is the answer to your question. Features are what matters to the majority of users. The ability to sideload apps itself is not.