Yes, this restriction is a huge PAIN. Back in my Windows Phone days, I ran development builds in addition to the store versions of my apps as dog fooding, or even just prototypes I never finished. Now I have this kind of apps on a secondary Android phone, but no iOS apps despite using macOS and having iPhone a main phone. Apple is really shitting on developers and hobbyists.
You barely can anyway. Unless you pay $99/yr for a developer account, any self-compiled apps you install will expire after seven days. If you want your app to keep working, you need to plug your phone into a computer and recompile every single week. That's just not realistically usable for anything other than simple testing.
Yes, it should be allowed. But it's not. It's been this way for the entire history of iOS and it's awful.