Why do asteroids explode high in the atmosphere?

Phil Plait:

If you’ve ever watched a bad movie about an asteroid hitting the Earth (and there are so many that if I have to see another one I swear I’m rooting for the asteroid) then you might have noticed that the asteroid itself always slams into the Earth’s surface, and it’s the impact itself that does all the damage.

Actually, you may not have noticed, because it happens so often and is such a trope that you might just take it for granted, like the air you breathe.

Oh, but that air is important. If the asteroid is small-ish, say a few dozen to a few hundred meters across, then air is critical. That becomes obvious if you turn away from the silver screen and instead watch an actual asteroid impact … like the one that happened on February 15, 2013, over the city of Chelyabinsk, Russia.