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You are absolutely right about cars not being aerodynamic. People buy cars because they look good. Just look at a 747. It is pretty blunt in the front and gradually tapered in the rear, like your streamlined shape example. Airlines don't care what a plane looks like, they want efficiency.
I'm not an aerodynamics expert but I did take a graduate level aerodynamics class while in college. The idea of streamlining is to accelerate the air around the body, and slow it back down again at the rear. When the air is slowed, its pressure increases. High pressure at the rear of the vehicle is a good thing. You don't want flow separation because when the flow separates from the object, you don't get pressure recovery beyond that point and are left with a large low pressure zone. The low pressure zone "sucks" the vehicle backward (drag). Keeping flow attached at the front is easy. That's because the flow is accelerating from a higher pressure area to lower pressure area. Keeping flow attached at the rear is tricky and pretty much impossible to keep fully attached. That's because the flow is moving from low pressure to a higher pressure (kind of like water flowing uphill... it will do it but not gracefully). If the surface is too steeply angled or changes direction too quickly, the flow will separate. The energy of the boundary layer will be shed in large vorticies and dissipated as heat instead of pressure on the rear of the vehicle.
No matter how streamlined a shape is, separation is going to happen somewhere before the trailing edge. So if the shape is cut off at that point, aerodynamics won't be affected. Many cars probably separate flow at the rear window, so what the body looks like after that won't matter too much. If the car is well designed and can keep flow attached further rearward, the shape of the trunk area will be important.
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