They spent years and untold amounts of money on the body design. They cannot change it without affecting their drag coefficient and killing their max range. The body design is part of the magic that makes the Tesla possible.

edit: drove my friend's Model S down the highway at 75mph and put down the windows. No air entered the cabin. We sliced through the air like a hot knife through butter. It was remarkable. Also consider that they are legislating to get the requirement for side mirrors removed. If they replace them with cameras they gain 7% more range.

afaik Tesla is the most aerodynamic road legal car in the USA