[Alan]

I think you're correct, Kontan. People could live in Chernobyl already. The radiation is higher than in any other place but not enough to cause birth defects.

But think of it this way. There are three things that make the Fallout universe different than ours:
Physics follow 50's sci-fi. Radiation causes rapid mutation in species.
Microprocessors were never invented. Machinery got more complex and more advanced but required much more energy.
Culture also follows the 50's. There's a "better dead than red" attitude that translates into even more rampant xenophobia than the real 50's America.

So couple that with a nuclear holocaust and you get a plausible scenario.
The real damage came with the fallout rains that killed nearly all animal and plant life.
Vaults opened around 2100. FNV happens in 2281. Most people were too afraid of leaving the Vaults, and so we have the plot for the first Fallout.
That main character founded a successful community from 2162 to 2167 and his grand-grandson is the guy from Fallout 2. This game happens in 2241 so there's not that much change between 2,3, and NV.

For the following 60 years after the opening of the Vaults, there were already groups and tribes forming, most notably the Great Khans. Half a century is a decent amount of time to build nomadic structures from scratch.
Most of humankind's (or America's) legacy comes from Vault 15 which was overcrowded and struggling by 2261 (what's with all dates having a 1 in the end?).
In 2196 it evolved into the New California Republic. 50 years forward and it has a big agricultural economy with its own social class of landlords, or more specifically big-name cattle ranchers.

There are 85 years left between that and FNV. Caesar was captured in 2247 by the Blackfoot and became its leader and incorporated the other tribes into his Legion in a matter of years and by 2271 all 86 tribes from the Colorado and Arizona. This empire took 24 years to be made.

Fast forward ten years and you get Fallout New Vegas, with the only city which managed to survive intact to the nukes. Mr. House was off by just one day to save the actual city but its core survived.
Why hasn't THAT grown into a full-fledged city when it already had all the necessary reasons? Because charming and intelligent as Mr. House might be, he is a despot. He is not concerned with the people of New Vegas; he is only concerned with New Vegas. This is obvious when you see Freeside and what they have managed to build and call a city. Mr. House merely tolerates it.
In a very realistic scenario, the Mojave wasteland suffers from a phenomenon that other places don't have, which are the Fiends.

Los Angeles had the Master, but you get one lucky bastard to destroy him and everything he created is destroyed.
The West overall has the Great Khans, but they still have a sense of identity which makes them abide to certain codes.
The biopolitics of New Vegas, however, perpetuate the Fiends. You invite people to vice, trying to fish for their money with drugs, sex, and gambling, but you immediately expel them as soon as they're down (it takes 10,000 caps to be allowed into New Vegas).
You get a bunch of violent junkies looking for a way to survive and a gang that unites them in their desperation which won't be pursued by Mr. House as long as they don't get any closer than Freeside.
New Vegas is safe within its boundaries, but none of its people are outside of it. New Vegas is a machine, not a city.


So the societies and the politics aren't that realistic. Many factions enjoy lands larger than most current European states. NCR has all the southwest, Caesar's Legion Colorado, Arizona, and presumably New Mexico, The Enclave claims legitimate authority over New England, and the Great Khans technically span throughout all the Midwest as a legitimate but nomadic empire, especially after New Vegas. Frankly it would be more surprising that The US would ever be united as is.

The beef is more of the destruction. We get why they wouldn't fix the highways, but how can they have markets from pre-war scraps 200 years later?
What interests me most is what happens with places like South America and Africa? They must have suffered horrible environmental disasters but there's no reason any nuke was targeted at them and Fallout doesn't reach that far. You probably get a place that has no Post-WWII technology but that lacks mutations and still probably enjoys havens of genuine rainforest. That's the only way the earth could still survive in producing enough Oxygen, and I'm pretty sure Bethesda has enough lateral thinking to eventually incorporate that into the canon.