EU antitrust chief to Tim Cook: Apple must allow third-party app stores::The European Union’s Margrethe Vestager has met with leaders of US Big Tech firms to discuss their operations in the EU, and with Apple’s Tim Cook concentrated on the App Store and Apple Music.
If you look at macOS, you see that even apps not installed through the app store need to be signed from apple in order to not trigger a big ass warning. Apple on it’s own proofs that the security aspect is not an argument. Edit: I think windows does this as well to some degree