It'd be too cumbersome, there'll always be a new 'hack' to install a different package, then there's the ability to inject your messages and to send over these

It'd be too cumbersome, there'll always be a new 'hack' to install a different package, then there's the ability to inject your messages and to send over these 'approved' apps and little to no way of ensuring such a huge infrastructure are all up to date on 'security' patches which stop these exploits.