But there's nothing really to gather in the Mojave.
-The city of Vegas is impossibly leveled compared to every other major metropolis area from L.A to D.C., so there's no chance of them eating the pre-war food people in D.C. sometimes eat.
-There are no remnants of old tribal farms anywhere in the game, nor does anyone mention such a thing ever existing. IIRC, the all roads comic called the tribes nomadic, which means they wouldn't have had farms, as those require you stay in one place.
-There is less edible animal life in the Mojave, you got geckos and fish.... and that's about it.
-Trying to eat the plants only would result in the problem of depletion i mentioned before.
This is the problem with NV. its a world that only works because the NCR is there during the game, but it couldn't have worked before them because nothing would be there without them except the Goodsprings farms. Everything before the NCR showed up is basically a giant plothole.
Fallout 3 may not show enough food production, but it establishes that things like trader caravans existed even 60+ years ago, just as Megaton, the first major city in the area besides maybe Underworld, was founded.