rear spoilers on personal vehicles are for show. They were adopted from race cars as sporty expensive extras to sell the general public. If you notice, Indy and formula cars have spoilers in the front and rear to provide down force for lightweight race cars racing around flat surfaces. The down force gives them grip in the corners and they are able to maintain the higher speeds.

Notice also that you don’t see much “drafting” in the corners in Indy and formula car races. Part of the reason is that when one car gets very close to the rear of another car, it takes all the down force off the car in the rear. In other words, the air is deflected over both cars, and the trailing car loses grip on the raceway surface.

The cars don’t travel close enough together, the cars don’t travel fast enough, nor are the highway curves sharp enough for spoilers to provide any value, except expensive aesthetic value.