[quotheth]

> Why should an application developer implement a sandbox?

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.