If you have to make a general statement I believe it has to do with the primary purpose of the establishment. Restaurants, sports bars, and dance clubs can claim the primary purpose of their establishment is something else than serving alcohol. Bars would have a very tough time claiming their primary purpose is something other than serving alcohol. Clearly the primary purpose of an airline is to fly you to a destination, not to serve you alcohol.
If you want a better comparison you should compare why you must be 21 to be a CRC member or PC member (I'll exclude NW's WC since they have self serve bars) but you can buy a FC ticket on those same airlines at any age. Both the Clubs and FC will serve you free alcohol. I believe the difference is the local clubs must adhere to local laws and regulations since they are on the ground.