The problem is that both AppArmor and SELinux[1] run applications unconfined by default. Also, sandboxing GUI applications is more work than just slapping AppA

The problem is that both AppArmor and SELinux[1] run applications unconfined by default. Also, sandboxing GUI applications is more work than just slapping AppArmor or SELinux on top. First, users still expect to be able to open arbitrary files from their home directory (which would then be added on a case-by-case basis to the application's sandbox). Second, X11 does not provide UI isolation, so a sandboxed application can still read all keystrokes (and thus credentials). The Flatpak people realize this and are working towards the goal of eventually sandboxing GUI applications (on top of Wayland). Apple actually nailed this pretty well. All applications in the App Store are sandboxed. If you open a file in such an application, the 'Open'-dialog runs out of process and links the opened file in the application's sandbox. Unfortunately, there has been quite an organized campaign by a subset of macOS app developers to discredit the App store and especially sandboxing. Sure, it would be nice if macOS supported more fine-grained permissions and Apple has been slacking there. But for the end-user sandboxed App Store applications are much more secure than downloading unsandboxed applications from random developer sites. As this and previous incidents have shown. [1] Since the switch to the 'targeted' policy.