[takluyver]

In some cases it does request permissions when you try to do things. E.g. sandboxed apps without full filesystem access can open & save files through a 'portal' - the system presents the user with an open/save dialog, and the app gets access to the file or folder they select. Things like screen sharing can work similarly - the system asks the user to approve access for the app.

But, as other people have said, this involves new APIs. In the open/save case, the app has to request that the system display a file chooser instead of drawing its own. GTK & Qt can hide this change so apps built on those toolkits benefit automatically. But beyond really self-contained things (like games) most apps that haven't been developed with a sandbox need some adjustment to play nicely within a sandbox. And making changes depends on building a consensus that it's worth doing.