Path of least resistance. It is easier for air to go around the nose of your car than to go through the grill, header panel, radiator, etc. What air does enter hits the back of your firewall and travels down and under the car. I think this is why most firewalls have a tapered or somewhat rounded firewall to floor board transition. In more modern cars a lot has been done to increase this flow, getting it where is need to be and getting it back out. Your radiator should be the only air access to the engine bay from the nose of the car. At low speed air can pass more easily, but at high speeds it doesn't. I think if you took a fan and put it next to a window screen, place it on low, the air will pass and the screen will not deflect, put the fan on high and the screen will begin to deflect and blow around the screen as not all the air can pass, the nose of a car acts the same way.
Under the car can be a different story, as you know it is not flat, plus stuff hangs down and causes turbulent air friction. Chin spoilers create a wake of sorts to limit air from entering the front and sides, to a point.
