I blame Fallout 3 for this. It's absolutely ridiculous for humanity to have done absolutely nothing in the 200 years since the war; one of Fallout 3's problems was that it felt like it was set around the time of Fallout 1, not two centuries after the war. There should be a lot of rebuilding and populated cities by now.
I think an exception can be made for the Capital Wasteland, seeing as it would have been one of the hardest-hit areas on the entire planet. I'm surprised anything is remaining standing at all there, to be honest.
Pretty much everyone currently living in the DC ruins either came out of a Vault, or moved to the area from nearby. Doesn't seem like anything really survived at ground zero except for the Ghouls.
Sure, it's ridiculous for no rebuilding to happen whatosever, but when the nation has descended into complete anarchy whilst simultaneously being blasted back to the Stone Age, it's very understandable for the DC Ruins to be in the primitive state they are.
Remember, Rivet City is the largest and most powerful settlement in the area, and it's only been in existence for forty years. Same goes for the Citadel. People just haven't been alive in the DC ruins long enough to build anything significant.
Regarding Chernobyl, there's a HUGE difference between a reactor exploding and throwing radiation over an area, and a saturation nuclear bombardment of an entire nation. DC probably took multiple direct hits from nuclear warheads; Chernobyl is simply not a valid example here. It was certainly a great tragedy, but a global thermonuclear war is on a whole nother plane of destruction compared to a single reactor overheating and blowing up.