I’d look at it from a slightly different angle. I’ve never seen it said anywhere that you would be completely alone (that doesn’t mean it hasn’t been), and it would stand to reason that if one person could still be alive, so could others. The world is a wasteland now, and with other survivors being around, then someone with some savvy would set up an outpost for trading. That to me is just common sense. Doing quests for those traders and/or factions also makes sense because then you are getting on the good side of the factions, and are under their ‘protection’ (i.e, not being shot on sight), or improving your standing with the trader, so he/she’ll be more inclined to give you a good deal and not rip you off (as much), or even so that they’d even want to trade with you at all.Personally, I don't like the idea that the traders exist and I never use them; and I really dislike the idea of going and doing quests for factions - to me, that goes completely against the grain of it being a survival game. It's bad enough in A16 in that there are some items that can never be found, only bought. I'd hate to think that in A17 even more stuff will be gated behind the quest system so that I'll be forced to be running around doing errands for people if I want to progress in the game. It completely destroys the idea that I'm out there on my own trying to survive against the odds.
In a survival situation, there will always be things you’ll want/need, that you can’t get yourself for instance. Being able to trade for that item might just be the difference in just surviving or thriving.