When talking about cars made specifically for racing this is the case. When talking about street cars there are plenty of customizations that can be made within a certain class. And that forces trade offs. I.E. speed or acceleration. Better tires or better suspension. It ads a lot of diversity to the races.the tailoring can come from tuning. but beyond that there's not much room to 'upgrade' within these classes. as far as the manufacturers are concerned, they've designed these cars to be as good as they can be, while still fitting into the regulations of their class