High volumes of travelers siphoning through the same paths, touching the same surfaces, interacting with the same service personnel.
Those travelers being from all-over, thus introducing new region-to-region microbiota mixes. Those travelers often tired or dehydrated from long plane travel across time-zones.
Even a heavily-trafficked park, grocery store, or DMV-like public service outlet might not face the same volume of widely-exposed & weary people as an airport does.
Who says an airport is any more risky than public life in a city? There is no personal interaction between travellers at all.