Probably because, outside of booze, fume and smokes on non-EU flights, the prices are exorbitant. Ever been in want of a pair of socks, undies, a tie or a pair of trouser, and found the airport shops your only option? And don't even get me started on the food and drinks.
But it's no wonder the prices are so high; rent per square meter at a major airport, will usually exceed by a large margin, that of the high street in the city it serves.