[0] - most desktop software doesn't need Internet access; even most mobile software doesn't need it in principle, it's only because of lazy business models that they have to have it.
OS X solves this quite elegantly. If an application is sandboxed, it cannot access files outside the sandbox directly. However, when an Open (or Save) dialog is requested, it is handled by a privileged daemon outside the process (it used to be pboxd). This daemon then makes the file or directory available in the sandbox after selection.
You still get app isolation, but it does not impede the user experience. (Of course, it does not apply for every scenario, but sandboxing is not a requirement outside the App store.)
E.g. Finder replacements, tools like Alfred, BootChamp, etc.
Maybe you misunderstood my post but that's exactly what I meant should not be misunderstood as sandboxing. Sandboxing api calls is not the same as hiding the file system.