Because of the angle of the Windshield....
The air will hit it and go up and down..... the air that is going down is a very high pressure and is then forced into the rear opening of the hood scoop....
Try towing your car on an open trailer with a light weight plastic bag taped around the body..... you will see exactly where all of the air goes....
Any air pressure under the hood will evacuate out the vents.... along with the heat
Morris