advertisements on roofs

As I was flying to California this weekend I was doing what most people in the window seat do look out the window. On the decent to the airport in Houston Texas I saw many buildings and could even make out what retailers they where. All of these buildings had one thing in common blank roofs, which were bland and not very interesting. This made me think why do companies not place advertisements on their roofs. Advertising on roofs would be a great idea, especially if the retailer is located near an airport. Many airports have over 50 flights departing a day. Most commercial planes have 25-35 rows, which would make 50-70 window seats. Then you have the people in the next seat over looking over their shoulder. The retailer could put something as simple as their logo on their roof and hundreds of people a day could see it. Business people who travel on a regular basis would see the image the majority of the time the travel. If the retailer did it across the continental US they could be known as the company that advertises on the roof. If you can advertise on cars, planes, and sides of buildings then why not on the idle space on your roof.