Something not really discussed but almost certainly a factor in Apple's reasoning is the blowback if someone vibe codes some type of app which creates a scandal, and it's reported as if it's an app available on the App Store.

Look at the recent controversy with Grok on X as an example. Imagine BBC runs a story with the headline "iPhone App does {very_illegal_thing}", or "iPhone users can now do {extremely_morally_objectional_thing}", and then idiot governments start trying to regulate or issue fines. One reason for the rule is so their review process prevents these things from happening.