I don't know if there's anything "special" per se (and more power to you if you want to walk to airports), but I can think of a couple of reasons why they do behave differently:

• Airports require substantially more land than bus or railway stations -- especially compared to some large central city railway stations where basically everything but the main concourse is underground, airports require large amounts of land on the surface, which often has the effect of blocking off the walkable parts of airports (the terminals) from other parts of the city. The noise and pollution can also make it unpleasant to walk.
• The average air traveler (on average! not the "HBO only" flyertalk crowd) has more luggage than the average bus or rail traveler, especially compared a lot of rail and bus travel doesn't permit checked luggage, so people often get naturally limited to what they can carry on their own. Because of their luggage air travelers often need to arrive by some kind of motorized vehicle (whether it be a train, bus or car).
• Airports generally "came of age" in the automobile era, where it was assumed that the majority of people accessing them would be by private vehicle, so little thought was given to pedestrian or even transit paths.
• Also because they came of age later than bus and especially railway stations, airports also tend to be located on the outskirts or in non-central parts of the city, so fewer people are willing to walk there.