Good question, but it's an easy answer. They hear the planes coming long before they see them. Afghanistan doesn't have a sky full of planes, and they can very effectively identify anything flying as ours.
Actually, the Brits and the Germans used these crude ground link systems in WWII. Very simple and effective. In WWI, they Allies used the same idea to give Paris warning whenever the Germans fired the Paris gun. In that case it was a telephone, but there was enough time to here 'incoming' and run into a hole because of the very high trajectory.
In a sense we are not really carpet bombing, as the planes are going in one by one. It's not so much a carpet as a hallway runner. Perhaps that will change.