Because you can't render true black and rendering the world gives the developer more control over what the user sees.
I have no doubt that AR will power many "low end" gadgets like google-glass or snapchat's glasses, but for going full gargoyle mode, I imagine the ideal is a fully computer rendered landscape, even if it is mostly true to what you would see in the real world anyway (since it offers so much more power).
I have no doubt that AR will power many "low end" gadgets like google-glass or snapchat's glasses, but for going full gargoyle mode, I imagine the ideal is a fully computer rendered landscape, even if it is mostly true to what you would see in the real world anyway (since it offers so much more power).