You very much can build and sideload an IPA on iOS if you have (access to) a computer running macOS.
Sure that's beyond the reach of most people, but how on earth do you people imagine that people develop and test new apps on iOS devices if you think an app _must_ be published to the App Store to run at all?
I feel like I have this discussion every week. It’s technically possible but—by design—practically unworkable for apps you actually want to use rather than just test. They expire after seven days, at which point you need to reconnect to a computer and reinstall them.
Sure that's beyond the reach of most people, but how on earth do you people imagine that people develop and test new apps on iOS devices if you think an app _must_ be published to the App Store to run at all?