When driving, you are in a huge chunk of mass. Thus chunk of mass has opposition to its inertia, air resistance. Since we are not in outer-space out environment of air is comprised of different gases which have a density. It requires more and more force to overcome these particles.
In drafting, you have an object that is not connected to you that is breaking the air for you. Imagine putting your hand outside the car when your driving, not doubt you can tell a difference between your hand fully vertical, creating a wall, and fully vertical, creating a narrow plane. Now imagine taking your hand fully horizontal, creating the wall, and place your other hand behind it. You would have to provide some force to keep your first, horizontal hand, stationary. But, you wouldn't need to apply nearly as much force to the second hand, since the first hand is disrupting the air flow and breaking it up before it reaches the second hand.
I just got up, so I will re-read this when I am more awake
