Accessing stuff outside the sandbox becomes either complicated or completely impossible.
Complicated can mean less convenient for the user and/or more work for the developer, e.g. more expensive.
As an example, on iOS each and every app is sandboxed. That's one reason there are things which are common with desktop operating systems, but virtually impossible on iOS. Like a simple file browser - an app per default only has access to files in its own sandbox. You simply can't get at files of other apps. To mitigate that either the OS has to hand out access to individual files (like with the photo library) or files need to be copied from one sandbox to the other.