If the car does not have the parts available for purchase at Tune Shop and a standard part does not appear in the related category of the tuning menu, then it can be implied the car has a better than stock or available upgrade component already installed. There are measurable differences between stock, single plate, and twin plate clutch; and same for stock, sport, and semi-race flywheels; the catch being the most expensive part is not the one always best suited for the car. I wrote a tuning article on drivetrain optimization a few weeks back, you can read it from thread in signature link. I need to update that article now though, since custom transmission tuning has become available, matching gearing with custom tuned limited slip differential can counteract some of the adverse effects initially recorded; however, the performance metrics should remain largely unaffected.
I think the real reason is that it's not complete and could be patched in by an update. Just like ballast for example.
