Because why not add drag to your car.

Spoilers, air dams, whale tails, the wingy bits. Even though not traditional wings (or airfoils) whose job it is to produce downforce, such as an F1 car. Spoilers are meant to disrupt the airflow over the rear of the car to reduce lift and reduce drag. The 911 Turbo was a pioneer of this. When developed in a wind tunnel or with CFD spoilers can deliver impressive results. This however is not how 99% of fucktards use them. Most people seem to just walk into Pepboys and buy the biggest ugliest piece of crap they can find and randomly glue it on the back of the car.