i think the story might have meant to just make a fairly large patch for the player to be inside of and to feel otherwise cut off.
all normal services are severed. comms and travel systems are all kaput. the rest of the world might be in the same situation or not even there. Zimmer still gets where he wants from Canada though, and apart from him i can't think of another example now.
the world might be split into patches of survivors and that 'little' piece of the US is a tough enough place to just gather food and get basic medical help, and so on, without thinking about further mass destruction.
after all, most people have ended up wondering where their next piece of incredible power giving hat is coming from. or where to get another 3 million rounds for their quadruple power mega laser multiple moose launcher.
i suppose it was meant to come across as civilisation springing back up out of the vaults and rebuilding itself. i really don't know.
early on i stopped even trying to take notice of what would made sense on a global scale or of Fallout canon and just 'went with it' on a superficial level. i found it more fun to think of it like a big mental paintball game in a bashed up old theme park.