You sound like me when I'm attacking such issues with The Elder Scrolls...
The Fallout universe is decidedly different, however. There are two types of fiction, spread out among all genres:
1.) There is self-validating fiction. (The Lord of the Rings, Star Wars, many books set in the D&D universe, books written by authors such as Robert Jordan, L. E. Modesitt, Jr., R. A. Salvatore, etc.) These stories establish their own, fully-qualified realities. All events are logically explainable within the clearly established laws that govern that universe. There are no deus ex machina moments (unless clearly established "gods" or supernatural powers literally intervene).
2.) There is non-quantifiable fiction. (Films such as Monty Python and the Holy Grail, Looney Toons, TV shows like Lost or Heroes, books by Terry Pratchett, Robert A. Heinlein, etc.) These are completely groundless worlds created as little more than a backdrop for the true focus of the story: satire, social commentary, criticisms addressing cultural norms, etc. They contain various fantastical elements that exist simply as symbolism or to highlight allusions or allegory -- not to establish a validated world.
TES falls under the 1st category. It consists of an entire creation myth, various cultures, clearly established philosophies behind the concepts of time and magic... It's a world in which, if you ask: "Where did this come from?" there is a valid answer even for very ethereal ideas (such as life and death).
Fallout is the 2nd category.
a. How were we able to discover all of these nuclear secrets after WWII?
b. Why to vehicles look like a cross between 1950's era auto designs and campy B-movie sci-fi spaceships?
c. How could you possibly use radioactive material in everyday items and not suffer regular, catastrophic events or walk around in an irradiated desert without dying in weeks due to the fallout?
The game doesn't even try to answer these things, because the validated game-world answers would be...invalid.
Spoiler a. Scientists were more brilliant in the world...just 'cause.
b. There was a huge retro movement in the arts after nuclear power became common-place...just 'cause.
c. People developed a, um, resistance to radiation after being around it all the time...just 'cause.
Better to answer them by interpreting them for the groundless elements that they are -- just aesthetic "whatevers" there to draw a clear connection to ideas that make sense to us in the really-real world.
Spoiler a. It highlights the outrageous negligence that heralded the use of atomics during the late 40's and early 50's. It's the un-achievable dream...that we eventually abandoned in real life because nuclear power is just too dangerous in reality.
b. The vehicles are satirical imagery of the "idyllic American life" that wealthy white people of European descent strove to attain after WWII ended. (White picket fences, mowed lawns, an American built Chevrolet in the driveway, the man in a business suit and the woman in pearls.) It was nothing but a mirage that only very few Americans enjoyed, and it simply sheltered them from the reality of the world they didn't want to deal with. Now, we see those illusionary ideals rusting in the sand.
c. They can't die. Radiation itself is not real or realistically represented in the game -- it's a symbol of humanity's effect on the world: the lingering destruction we cause long after we ourselves are gone.
The Sole Survivor would, of course, adapt to living in this new world. Why can't he build beautifully crafted furniture? Same reason as others don't -- with what? Is he a master carpenter with fine, precision tools all of a sudden? And how would that theme fit in with the inevitability of the Wasteland? The whole point of the game is that we lost it -- the world -- it's gone. The future looks like endless war, and poverty, and a constant struggle to survive...so...well...nothing much has changed actually. But it's dustier! And now everyone is subject to it...not just the poor. The American Dream failed.
EDIT: Sorry, terrible conclusion. The end is that to add in too much realism would be to detract from the focus of the game being almost wholly symbolic. It's not meant to be a simulation of survival in an actual radiation zone; it's meant to be an engaging exploration of the human condition heightened by a gigantic, ridiculous "What if?" scenario. That doesn't mean that it can't be gritty and emotional, but it's more Gulliver's Travels than A Tale of two Cities, in execution.