Hey kind stranger who is supposed to do the garden while I go shopping, I really don't trust you. So to be sure you only do the garden and nothing else, here are the keys to my house, please ensure that every door and window is locked. Thanks.
The only other entity who could set it up for you, so every application automatically launches in a sandboxed environment, is the distributor, but then again it's your responsibility to chose a distribution that does that.
If you want security you have to do something about it at one point or another.
Leaving this to the user leaves the vast majority of users unsafe. This is an unacceptable state.
There is no way around you either taking care of that yourself or you choosing an operating system that enforces it for you, like Qubes OS.
Because they are the ones who understand the necessary capabilities of their program and the ones who have access to the source code...
> That's a huge waste of time and it's much more efficient if the operating system or the user enforces it instead by using existing sandboxing technologies like firejail.
Actually it's a far better sandbox when built into the program. And it doesn't leave users relying on installing arcane operating systems or becoming technically savvy.
> It is also untrustworthy and insecure, since after all you don't trust the application.
No, trusting the application is implicit since it's installed by the user. The sandbox exists to protect against a compromised application.