Also, if you go by the Fallout lore, shortly after the bombs dropped there was a "black rain" that fell for several days, and wiped out most species of animal and plant life. So according to that, perhaps the soil is still so toxic from the black rain that only a few hardy weeds and bushes can live in it..
This is one reason that didn't happen, another being (I'm sure it was said before but in case it hasn't I'll say it) that nuclear energy works differently in FO than in real life. It follows 1950's Sci-Fi nuclear physics, where when you irradiate something, it gets bigger and meaner, instead of like real life, where it grows tumors and then suffers until it dies horribly.
EDIT: As for why the buildings didn't collapse or erode, well.. you ever notice there's no rain in the game? For stone/concrete to get soft and fall apart, it needs a catalyst, most commonly H
2O, to start the chemical changes. Without that, well.. in a dry, arid environment, few things deteriorate fast, if at all. They've found ancient corpses with skin still on them in deserts in real life, so I don't see why the buildings in Fallout wouldn't remain even 200 years later if denied the things they'd need to erode.