Because while people might be OK having the same $1000 iPhone 6 as everyone else, they're not OK having the same exact $80,000 car. They want an extra feature ere or there that the next guy with a BMW m5 doesn't have. They might want some extra carbon bits here or there. Cars are as much fashion as watches, and car customization is a huge industry / widespread interest. If dealers / mfgs can capture even a bit of the aftermarket industry (think simple stuff like chrome exhaust tips / different wheels), it's probably a win.