In a nutshell, because an application won't be able to do anything evil. We're already halfway there on mobile devices. An Android app cannot access system files or files of other apps, period. "Run as admin" doesn't exist. It can't access shared files like camera photos or documents without explicit user permission.
This is mostly accomplished using SELinux, which is an afterthought slapped onto the original OS architecture.
There are exploits that defeat these walls, but it's getting harder. Walls built from the hardware level up will be almost impenetrable and might require finding an error in the chips' microcircuit designs.
