I asked this very question to an advertising expert (wife) and the response was," Normal people don't look at airplanes with enough interest to actually see the ads."
Unless you paint an airplane to look like a killer whale the ads will be missed. Also it's tougher to track how sucessful the ads are verses radio, TV, ect.
If your company had only so much advertising dollars per year where do you think it would reach the most people?
Personally I like seeing funny ads on planes. Santa Claus splattered on the nose of a 737 comes to mind.