Because it is like X-raying people coming through front door to check if they are hiding anything metal having opened and unsupervised kitchen door. Plumbers are banned, yet you can still be killed with a ceramic knife by a polite assassin or with metal pipe by a brutal crook.
The main problem is that it comes with a obvious loss of functionality which I doubt that can be leveraged with security gain -- the other thing is that it gives a faux sense of security to the user (i.e. yes, I have agreed for full file system access, but it was sandboxed, right?). Finally, the system is as weak as the weakest elements, and those seemingly lay in their own code (and can be fixed without hurting the functionality).
So if Google decided to remove the sandboxing functionality in the next version of Android and just run every app under the same user id, that wouldn't reduce the security of Android devices?
Again, I'm not against adding seemingly redundant additional security layers, I'm against calling sandbox an universal cure for everything and using it to constrain the user in morally ambiguous way. Android system is also damaged because it makes users to root the device to do things which might be done without such elevation of privileges and, what's worse, violating warranty.
