They have vaults and ruined buildings full of food and technology. All they lack is agriculture, which they don't have any need or possibility to develop. They have no reason to trek west, the direction the FEV mutants came from, just to develop low-tech agriculture.
Sure, maybe tribals came over the mountains. They wouldn't have anything to share! The Capital Wasteland survivors are already experts at surviving in their habitat.
"Hrr, maybe put this seed in ground?"
"No tribal we tried that, the water is literally poison. Now get the hell into our fortified walls and have some snack cakes, we have thousands."
"Hi, we're the Brotherhood of Steel! Have our technology!"
"This helps but we still can't actually *grow* anything. Though we'll start researching it anyway, because we're living pretty well off all these caches. Maybe in a decade and with a GECK..."
"Hi, we're like the earlier FEV mutants except we're giant humanoids with rudimentary intelligence. Hurr."
"Well, crap. Good thing we have walls and high tech weaponry to survive, for now, particularly with the Brotherhood's help."
Does it actually say anywhere there's nothing between the East and West coast worthy of note? Cause New Vegas kinda contradicts that. There's loads of tribes and stuff in the nevada desert, to the east of like a dozen cities from Fallout 1 and 2. And you'd think the East Coast would be the less barren coast.
The tribes of Caesar's Legion (which hardly count as Midwest) were isolated, agrarian tribes. Until an educated young man from the coast came through and reforged them into a mockery of the Roman Empire.
And the East Coast has a lot more buildings and resources... It was just bombed a lot more, because it had more buildings and people. Now it still has the resources and some of the buildings, but most of the people were successfully killed by the bombs. As intended.
As for animals, well, anything that isn't human. Molerats, deathclaws, radants, radscorpions, brahmin, those anthropomorphic mudcrabs. They have to eat something that isn't just meat. Else they'd have died out by then. And considering how big they are, they must find a lot of it.
There's apparently some sickly grass, and occasional bushes. Fortunately, in this setting radiation does some amazing things. Ghouls live off of it, FEV mutants (animals and human) are immune to it and might live off it too (particularly the screwy east coast versions, where the FEV is weird). All it really takes is one radiovore near the bottom of the food chain, like radscorpions. Or even Brahmin.