Why installing an app allows to "drain wallets"? Why does a video chat app, which is installed from untrusted source, have an access to a wallet private key? Why OS allows this?
Anything less, and the user will find a way to accidentally give admin permissions to random apps.
NB: I don't believe that it's the OS's job to protect the user from themselves. Shielding people from consequences of their own actions results in people making worse mistakes later on.
The kind of safety you're talking about is only possible in strictly walled gardens.