Attacking the Legion head-on would be a blood-bath for the NCR, they both had to recuperate their losses after the first battle - in-fact the NCR more than the Legion. Unless you believe that the NCR could defeat the whole Legion then keeping them largely bottle-necked behind the Dam in an endless stalemate is probably the better option, the NCR invading the Legion would probably go as well as Barbarossa. What does the NCR ending say? That the NCR secures the Vegas area, nothing about an advance into Legion land; how long before the Legion comes back under Caesar or Lanius?
EDIT: That's because Oliver's position is defensive and not offensive, like most modern militaries you know?