Because the mechanicals and bodyshell of all the nascars are essentially the same, the only difference is the paintwork. In modelling terms this would be known as the "skin" of the car.
I am well aware that modern COT NASCARs are spec-racers. However I fail to see how that will magically ratchet up the car count on the actual track in the actual game. Does anyone actually have some background in game design or are we just tooting "facts" that we picked up from a retail background?