But the question is why you shouldn't be able to sideload an app at all. Having a curated store is pretty cool, and you can have that while also letting users install their own apps manually (even with barriers, making it as hard as they like).
If Apple allowed sideloading but went out of their way to make it obnoxious enough that everything which could be would still go through the app store, you'd just have all of the exact same complaints about how Apple is deliberately making things painful for users.
ios Safari puts in place some limitation (sandboxing, constraint device apis) to guard against malicious actors, but it is not curated the same way ios App Store is.