If you look at the Mac as an example, the vast majority of software is still distributed outside the Mac App Store, usually because either:

- The software is free and the developer doesn't want to pay $99/year to Apple

- The software is paid but the developer either doesn't want to give 30% to Apple, or they want to use a pricing model incompatible with the App Store (discounted upgrade pricing, rolling subscriptions etc.)

- The software does something that violates the guidelines, or in general is incompatible with sandboxing (or would be a worse product because of it)

Because so many applications are not, or in many cases _cannot_ be made available via the Mac App Store, users of said apps are in a sense "forced" to install outside the App Store. I believe that if sideloading was feasible for iOS, many developers (and certainly the big players) would pull out of the App Store completely.

I want to make it very clear though: I don't consider this an argument against sideloading apps at all. I consider it evidence that the App Store (on both iOS and macOS) is woefully inadequate at covering the full range of software developers want to build, and that in turn hurts customers.

I would love to use my iPad for more work related stuff, but I'm a software developer, so most of my day-to-day work involves software that just cannot run on iOS. If sideloading was available I could actually use it like the "Pro" device it claims to be, rather than just a very nice content consumption device.