Cause the new cars have way different headlights also, other then wheels and headlights its all the same. People don't change headlights cause its not easy to do--or they would along with the wheels.
You're exactly right if headlights were easy to do everyone would do them. Cars with 2 headlights are easy because of the kits that are available in the Aftermarket. But when you get into cars like my Cordoba which has a 4 headlight system there is nothing out there. For a good reason there isn't much demand for them.