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They're there. It's game logic. You don't see them for the same reason NCR is only three maps, only has 1 councilor/senator, and only about 40-50 people in its city limits. That's why the Chop Shop in Reno exists, why the bum outside of NCR offers to watch a car for you before you show up in one, and a reason that NCR built a garage in Shady Sands. So to explain "game logic" in this instance, there's nothing precious about building a car of your own if you can steal one – or if somebody else in town has one. Or in the words of one designer (me), "there's no good reason why a PC would want to undertake a fucking huge Fed Ex quest to rebuild one if they can jack one from the locals." The last part is especially true considering town-wide mass murder is possible in both Fallout 1 and 2. And before you get the image of tanks and jeeps flying around everywhere with heavy machine guns mounted on the back, most of that junk is old tractors and crap like taxis, old buses, snowplows, and even old construction equipment. It's possible that mysterious old steam-truck mentioned in the bowels of the F1 data archive is still lumbering around somewhere. The caravan houses of the Hub, in particular, around the time in Fallout 2, have been looking to further its trade influence, and new vehicles (and types of transport, such as trains, boats, or barges) have been eagerly sought after for carrying large amounts of trade goods vast distances. Good ol' human greed will move mountains. Or at least rebuild things that can. Once they learn of the Enclave's presence in the North, they are likely to have huge bounties promised for vertibird plans - or better, a working vertibird.
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). actually to produce food, someone should be transform into trog first and slaughtered and cooked though. I don't think nobody want to buy or eat slop of trog. 
goodspring is way better trading hub than whole DC wasteland I guess.