Most of the time, you can put your board in the car.
My theory is that snowboard racks evolved from Ski racks. It is not easy to fit 180-210cm skis in a car, truck or suv. But then it became kinda cool to have ski racks on your car. So when snowboarding became popular, people used ski racks and had the same mentality. If you are a real baller, you keep your boards in the car or in a box on top.