Another way to look at it is that this racing game looks so great for the very reason that it is a racing game. Cars do not require the same polycount and level of detail as human models do to have the same realistic appearance. The same thing is true for roads and mountain surfaces, especially in a game at zooms you past it all, and this zooming through the game also means you don't need detailed trees and other scenery. There are also no real crowds in racing game, or at least any crowds that you can approach closely to observe their level of detail. It's not like Assassin's Creed where you can walk up and look a man in the eye, face to face. Racing games looks great because they can get away with cutting corners in places where other game genres cannot do so without having artifacts and imperfections stand out like crazy.