Co-Writer/Producer Roberto Orci has your answer here.

TrekMovie.com: So what is your guys logic for setting it on land?

Roberto Orci: Besides the thematic stuff we discussed, which is to connect it to today and make it clear. Firstly, there is the notion that there is precedent in the novels, etc that components of the ship can be built on Earth and assembled here or there. And the second thing is that the Enterprise is not some flimsy yacht that has to be delicately treated and assembled. The idea that things have to be assembled in space has normally been associated with things that don’t have to be in any kind of pressure situation and don’t ever have to ever enter a gravity well. That is not the case with the Enterprise. The Enterprise actually has to sustain warp, which we know is not actually moving but more a warping of space around it. And we know that its decks essentially simulate Earth gravity and so its not the kind of gravity created by centrifugal force, it is not artificially created by spinning it. It is created by an artificial field and so it is very natural, instead of having to create a fake field in which you are going to have to calibrate everything, to just do it in the exact gravity well in which you are going to be simulating. And the final thing, in order to properly balance warp nacelles, they must be created in a gravity well.