Time for me to dust out my big book of Fallout lore. This is considered Canon as far as the Fallout universe is considered.
1. The only reason anyone in the United States survived is because of the very spread out population, and the Vaults. This does not preclude some other countries with a spread out population to have some survivors, you won't have anywhere near as many as in America, because of the 128 vaults.
2. The would is isolated, so there is no telling if anyone on other continents are alive. Even the Pen and Paper DM book isn't telling.
3. There are gigantic radioactive tornadoes running through most of the Midwest, plus the Rocky Mountains make travel from on end of the US to the other very dangerous.
4. Chicago was mentioned briefly in Fallout Tactics, as part of a coast to coast sweep in a massive Fallout game, so it might get a peek in the future.
5. If New Vegas is anything like New Reno, then Fallout:New Vegas could possibly get an AO rating. If you're not sure what I'm talking about, Fallout 2 is for sale on gog.
6. The Brotherhood of Steel you see in Fallout 3 are part of the group from Fallout Tactics that remain behind. (A fitting little nod to Fallout Tactics if I do say so myself)