The thing to remember is that they are probably mixing actual footage(w/ pyrotechnics and whatnot) with CG. An example would be in Dinosaurs with the background bieng actual and the dinosaur and other objects being digital.

If you were to recreate the world(with tree include files that have many branches, radiosity, media, plus any isosurface) it would take a tremendous amount of time to parse and render. I'm talking about scenes that would probably require a better computer (at least better that my worthless Compaq). Such computers as those nice $20,000 SGI's (a little out of my price range)