It also has persistent permissions.

Think about it from a real world perspective.

I knock on your door. You invite me to sit with you in your living room. I can't easily sneak into your bed room. Further, your temporary access ends as soon as you exit my house.

The same should happen with apps.

When I run 'notepad dir1/file1.txt', the package should not sneakily be able to access dir2. Further, as soon as I exit the process, the permission to access dir1 should end as well.

A better example would be requiring the mailman to obtain written permission to step on your property every day. Convenience trumps maximal security for most people.