I think that the GP4 graphics engine was designed for one or two more games in the Grand Prix saga. Surely GP5 and GP6 (perhaps in 2004 and 2006) would use the same base. The quality of the models (polygons, vertices) and the size of the textures were totally under-dimensioned. It was according to the PCs and graphics cards of the time. But with enormous expansion capacity (today in 2020 we are converting Assetto Corsa cars, in many cases without having to resign anything in quality) or tracks with quality unthinkable 15 years ago. The original cars.wad weighed 23 mb and today we can use (if necessary) a cars.wad 20 or 30 times heavier! Unfortunately the project was truncated ... but we are left with the graphics engine to continue exploiting (and the unique AI). I think that is the explanation.