Maybe since mulitplayer is supposed to revolve around customization. Just my opinion. I really don't think GTAO will last long, though.
Well it isn't just those sort of customisations. Car tracking, you can see your car on the map wherever you leave it and come back anytime, and lets not forget we can insure them, if the car is impounded you actually have to go get it back rather than just pay and there it is.
Why can't we do these things in single player?