I don't think there is any actual architectural reason, just aesthetics and design

Places with very high ceilings feel more roomy than ones with small ceilings, and airports get very crowded, so a more roomy feeling will make passengers feel better about the space.

Also you, as a passenger, only see a small part of the airport. There is a whole other ecosystem of people working and running the place, usually on floors above the bottom floor, which means the cieling is higher to allow extra floors