I guess you misunderstood... it is not at all about sending stages to the Moon, to be assembled there, then launched. The point is to not have to launch great masses from Earth in the first place, but to build them on the Moon, from Moon materials, to fuel straight from there, and then launch. Or if it's not the Moon, then it will be asteroids.
Of course, this requires bootstrapping an industrial base from as small an invest as doable, i.e. sending robotic craft to start bringing in raw materials, and getting stuff done with them.
The trick is of course to figure out the details. Many ideas were already written up by Gerard K O'Neill back in 1976 - read "The High Frontier". Technology has since advanced, and there have been many proposals since. Private companies are digging into the problem, there are government invests.
The goal also is not to get to Mars, but to get mankind into Space, not just on a few short excursions, but for good. Moving heavy industry up there might also solve some problems down here (think energy, pollution).