It's worth pointing out that F76 is based only what, 20 years after the fall? And everything is already royally fucked on so many levels out there. So it doesn't seem unreasonable to me to see those various biomes dying out. Also, a quick google search says that it's roughly 330+ miles from DC to West Virginia (varying a bit depending on where in WV you're going). There is no way anyone in DC, other than the brotherhood/enclave, have the resources/infrastructure to manage that long of a journey, and not die. They are lucky to survive just going a few BLOCKS to scavenge for supplies, without dying. And we know this, because as the player, WE are lucky to survive moving a few blocks at best, without running into a host of lethal threats. So they are limited to the nearby area for resources, all of which are blasted.Fallout 76 shows a West Virginia with plenty of living, flourishing trees. Fallout 4 shows Boston surrounded by forest. So there are trees and they are quite near by modern standards. Fallout 3 contains one (dead by the time you find him) former Enclave member who went from Navarro to the Capital Wasteland, Harold who travelled from New Reno (and had gone there from the Hub in between Fo1/2) and both a synth and a synth hunter from the Institute in the Commonwealth. DLC has you going to the remains of Pittsburgh and Point Lookout, which looks a whole lot like Louisiana but could be Georgia in a pinch. Fallout 4 has you going from Boston to Maine in the DLC. All these games also show you traders, general wanderers and explorers roving the wasteland, so people obviously do move about quite a lot.
This really is one of those cases when you shouldn't try a Watsonian explanation, because Fallout 3 is pretty inconsistent in its portrayal of how people survive in the Capital Wasteland. People hide in old museums in a bombed out city without any way to get food or water yet still manage fine. Other people get their water from groundwater pumps pumping straight from water irradiated by a damaged nuke yet don't suffer tons of ill effects. The explanation is purely Doylist, in that Bethesda was going for a pulpy adventure in a post-apocalyptic setting and wanted lots of eyeball kicks and cool settings instead of a grounded simulation of how humanity would rebuild by making lots of farming settlements far away from the useless ruins of the old civilization.
As to the comment about FO 3 being inconsistent about irradiated water. I would disagree there. While every character isn't scripted to show radiation sickness, your dad clearly establishes right up front that the water situation is terrible, for everyone in DC. And you can often run into wastelanders outside the communities, begging for pure water, because they've been drinking irradiated water so long, it's making them sick. I always assumed that everyone else will, eventually, get to that point. But we are seeing them in the "still haven't had so much to be visibly ill" stage. Of course we don't see any actual passage of time in the game, so it's hard to say. But ALL water sources in DC are irradiated, not just the Megaton puddle. So those wastelanders are the stand-in for "this is where the DC area will end up eventually, if Project Purity can't be fixed"
That's my take anyway.