Because it is easier to do. A new car is a clean slate. Run it through and class it -- that is what gets done. Can take a while (see ITR V8s) due to arguing about subjective factors though.

What I'm scairt of is trying to do this with 300 cars. At once. To me it is just as likely that in all the noise that debate will generate -- and there will be a ton of it, all kinds of "me" noise -- we are going to fark things up as much as "fix" them.