it is because the air cabin pressure is lower (it is the equivalent of being at 8000 ft) and it is dryer then normal. Not to mention you usually need an hour on both ends of the flight for taxiing, boarding, complimentary TSA groping, and boarding. Then there is the fact you are in an enclosed space with lots of other people and can get some minor immunity issues with whatever bug they might have or leave on the plane
BOW DOWN BEFORE the Official Forum Pun-isher
Official XWA flight instructor
get busy with it!