Do people really tip pizza guys significantly less? What's the difference between them and waiters? Both bring you your food, both can spit in your food if they remember you're cheap.
That's messed up, you're essentially paying for the gas you would've spent to go pick it up. If you don't, then that guy is paying out of his own pocket to drive your food to you, and that's not even taking into consideration the extra wear and tear on his car.