Since we don't know why they aren't there we have to pick either Bethesda or Obsidian to blame.
And since Bethesda could have delayed the game for half a year so Obsidian could've had more time to fix things I'm gonna blame them.
By the way, this could have been in General Discussion, just saying.
I think it was pretty clearly Obisidian, as Bethesda games (including their last iteration of Fallout) have random encounters. No random encounters was a conscious design choice here, and seeing how many NPCs they placed by hand (and how quickly a mod to liven up the wasteland has been made) I can't imagine it would have taken an unreasonable time to implement if Obsidian had wanted to do it.
Fortunately, this is easily fixed with a great mod-http://www.newvegasnexus.com/downloads/file.php?id=38623
I've seen some ncr patrols 1 killed the legion camp for the quest booted on level 3. one ambushed some legion attacking a caravan
Not random, all preplaced. Go into the NV Geck and take a look at the NPCs; they're all preplaced, down to the last BoS paladin and NCR trooper.