Sorry for chiming in late, and apologies if these facts have been covered before.
1) Prepackaged prewar foods
Keep in mind, you're dealing with a In Game Universe that adheres to the Laws of Physics as determined by the 1950s "World of Tomorrow" view. Since prepackaged foodstuffs were just beginning to take off in the 1950s, a lot of people didn't grasp the fact that these couldn't remain on the shelves forever. Fallout 3 just takes this belief one step further. As for why there's still so many foodstuffs after 200 years...Washington D.C. was home to millions of people, after the Great War a few hundred remained. Believe it or not, even after 200 years there'd still be plenty of food to go around.
2) All the foliage is dead
For this argument I would like to point out our good old friend, Chernobyl. They had a nuclear meltdown at their power plant over 20 years ago. The gamma radiation killed off and sterilized everything for miles around. It's only been about 20 years and all the vegetation has grown back, overtaking the buildings people once occupied. I would think that after 200 years the trees will have recovered enough to where there would be insane forestation. Ah, but green trees don't look as desolate and hopeless as a barren wasteland, and that just kills the mood. Well, if they never came back, then all the trees we see in the game must be petrified.
Actually, the severity of the Great War was such that it actually froze Earth's climate in a state of perpetual, dry summer. Without weather cycles and abundant sources of water, it makes sense that only mutated varieties and desert plant life would be able to survive such a shift in climate. Even Vault City's greenery had to be constantly watered by its slaves.
3) Everything is still irradiated
Once again, Chernobyl. If the ground and water were still so irradiated that you could get radiation poisoning from it then do you really think that the plants and animals would have come back in force like they have? After just 20 years? Radiation doesn't sit around forever, it does dissipate.
Actually, the Radiation around Chernobyl is still there, it's just settled low into the ground - so you have to be extremely careful where you sit in Chernobyl (and why visitors still carry giger counters). Actually, the trees in the forests around Cherenobyl are actually drawing the radioactivity back up from the ground and into the air again - leading some concern that the area around Chernobyl might become dangerously irradiated again.
4) Ghouls
Have you SEEN real people who were exposed to as much radiation as the ghouls apparently were!?
Again, 1950s World of Tomorrow Physics.
5) Society at large has yet to move on
Seriously, guys, it's been 200 years, more than enough time for new countries to form. Not just city-states, countries. Let's look at the last quasi-apocalyptic event that happened to humanity which we survived: the fall of the Roman Empire. No, really, people back then thought that the world was coming to an end. How long did it take for new countries to spring up from the corpse of the Roman Empire? Yeah.
Several Hundred years, in fact. Also, the D.C. area is not reflective to the Country as a whole. On the other side of the country, the New California Republic is already well established by the time of Fallout 3. Given the D.C. area is actually
heavily urbanized, it makes sense that civilization is slow to return there. Even with the abundance of Prepackaged Food, the terrain does not lend itself to establishing communities, and likely does not how readily available sources of ground water. Indeed, all the population centers of the Capital Wasteland were there because they had access to Purification techniques. Smaller communities like Greyditch, Arefu, and Girdershade are ramshackle affairs that likely don't last more then a generation or two.
Oh, and the fact that individual people have yet to get over the fact that the US is gone is absurd. It's been 200 years, none of these people reminiscing about the glory days of America were even alive back then!
The Ghouls would have words with you. Despite the general prejudice, Ghouls are repositories for the knowledge of a bygone era. Eventually, those tales are going to be spread around to such an extent that people have a better grasp then you'd give them credit for of the United States. For that matter, to go back to the Roman Empire anology, because of the way Rome managed its territories, some of the more distant provinces still continued to operate as though they were under Roman authority up to a century after the fact.
And another thing, why are people still living in old, half destroyed houses after 200 years? People build, people repair, people rebuild. Societies do not set up in half destroyed ruins, sleeping on pre Armageddon beds for the rest of their existence. Megaton was neat, I liked it, but places like Canterbury Commons, Paradise Falls, and a whole bunch of other cities are just absurd. It's been 200 years, move on already!
In the Middle East, you have people living in buildings constructed hundreds of years ago. Just because it's old doesn't mean it isn't serviceable.
6) Prewar technology and weapons still work
The idea that machines never age and they can just last forever is an old, flawed, faulty science fiction idea from the 70s.
Well, since we're operating under the logic of the 1950s, I think this one's fairly self explanatory.
7 Electronics and lights still function
The fact that 200 year old light bulbs still work is extremely laughable, I don't care what type of bulb they are. I shouldn't have to explain why. How do I know that the bulbs are 200 years old? Think about it; have you seen a fully functional and productive manufacturing plant of ANY kind in the Wasteland? Who's building these new light bulbs? I also love these computers in old abandoned buildings that still turn on when you push the power button and operate like normal. Forget the fact that these things are primitive by our standards today, circuits still wear down given long enough.
I've got an even better question for all of you. Where is the electricity coming from? Last I checked all the power plants are in disuse, abandoned, and filled with feral ghouls. Where is the electricity coming from? Never mind the fact that a reactor has to be completely replaced with brand spanking new parts before it even gets close to the 200 year mark or it breaks down utterly. Parts break over time with regular use. Who is supplying power to a grid that has been in disrepair for a couple of centuries? And how does any of it still work. Do you know how much maintenance it takes to keep those grids working in the here and now when they haven't been hit with nukes? It is insanely difficult. And you think that the power plants will just keep on running 200 years after they've been abandoned? Yeah right.
And what about the generators that Tenpenny Tower, the Citadel, and other such places run on? Last I checked generators run on gasoline, and where does gasoline come from when society collapses? Nowhere, that's where. And don't tell me it's a nuclear generator. Let me explain to you the whole point of a nuclear reactor: it's to boil water. I'm not kidding you. The reactor superheats the water, and then super pressurized super hot steam is channeled through pipes to turn a turbine like a water mill, which generates electricity. Where is the water in these generators? Where is the cooling tower to re-condense the water to be boiled again? I'm telling you, these generators run on pixie dust! Pixie dust solves everything!
Once again, 1950s "World of Tomorrow" physics.
I can understand geeking out to the implausibilities of the game, but that requires disregarding the fact that everything in the game is build upon the assumptions of those of the 1950s.