Since the first Warthog they have is based upon Master Chief’s ride in Halo 3, and that just won’t do, now will it?
But yeah, Halo fanboys might recall the original, created by WETA (you know, the folks behind Lord of the Rings, The Hobbit, and the live action Halo flick, before it got sh*tcanned). But since the Warthog in Halo 4 has been slightly upgraded, MS thought it would be a good idea to have an updated, in real life model as well.
According to the LA Times, it was built by Aria Group, which specializes in replica vehicles for movies, and is built upon a Hummer H1. Yes, it really moves, but good luck trying to flip it back onto its wheels.