Having the option to install 3rd party is another attack surface, and gives a chance for the market - or authoritarian control - to to veer towards not being vetted by that walled garden.
Authoritarians are always going to be prefer authoritarian app stores where any app that threatens them can be swifty removed.
Authoritarians rule in part via suppression of information. All governments can mandate that specific things be or not be installed on devices, it’s typically only authoritarians that are afraid of unknown things being installed on devices.