Just to add to this already great answers, there's also engineering reasons.
Think about the massive space luggage sorting machines occupy. You just see a belt with your luggage, but in rality this could get to multi-story level, not to say that in some airports luggage travles from one ent of the airport to another. HVAC, plumbing, cabling runs, all need to be easily accesible by maintenence workers, so just for that it easily adds a conventional story just for that reason. Think all the behind-the courtain stuff... Loads of administrative areas that you do not see are stacked on top pf another in semi conventional ceiling height.