The conditions of the world are like that of the midwest dustbowl now with less rain. Farming is a terrible thing to rely on anyways. Unless you have the right seeds and know what you're doing, farming would fail miserably. If you want an example, then fine: Fallout 2. Arroyo. You're own tribe based it's food off farming and eventually started dying. Hunting is probably what kept them alive, afterall.
Also, it's like.... 5 caravans. with huge packs on their backs.
Also, Raiders and Slavers need to eat, so it is logical to capture or kill farmers. Farmers become slaves and thus become profit to buy food at the Pitt or from traders, depending on if they're bought. As far as Raiders go, Farmers are a source of food. Their products are, at least.
As far as I'm concerned, trade's more reliable in an area that's based in a major city's ruins. From a city comes lots to trade and many places to get it without getting into mutants, beasts, raiders, etc. Also, Mutants, Beasts, Raiders are all threats to FARMING considering all three seem bent on rape and pillage. Farming is also unreliable for a settlement to survive for 200 years... Megaton is the only one mentioned to have been around that long, next being the Underworld. In truth, there's no good explanation for how all of these places survived for how long.
Another thing to consider is Junktown and how it survived.. ON TRADE. Hub had crops, but based lots of income on trade as well. And this was trade with large distances, taking days to happen. Again, Fallout 3 doesn't show any outward connects, but considering some were mentioned It's not illogical to believe they exist. Maybe they were severed recently, who knows?
Also, hunting IS a better alternative. Why do you think native Americans survived so long before discovering farming skills? Is that stupid too?
And Brahmin can support a small settlement, partially at least. Combined with trade too
and scavenging!