Because it needs to be that long to have a low air resistance. There are two determining factors for air resistance. First, the cross section A, which determines the amount of air to be moved and second the drag coefficient c, which describes how well the air flows around an object. This gets lower the more "round" a shape is, but the sphere isn't actually the best one, you want the form of a falling raindrop. For that the shape needs to be quite a bit longer than it is wide. So if your car is e.g. 2m across, it needs to be something like 4m long to approach best efficiency. See also the empty "tails" attached to some racing cars for drag reduction.

More room for panels.