[jandrese]

Because there are no sidewalks out of the airport. This is depressingly common. Airport designers only think about cars and busses. Maybe the train if it is near a city that has it, but even that is far from a guarantee. Usually the only access is a high speed road that connects to an even more pedestrian unfriendly highway.

[johntiger1]

Exactly, some airports like LGA require you to cross a highway to get to any walkable area

[ghaff]

Boston Logan you can definitely hop onto public transit pretty easily. But I'm pretty sure you're not just walking to anywhere in East Boston from there easily.

[harambae]

Indeed. Silver line (to South Station, where you can transfer further) is free to get on at the airport, precisely to encourage riding public transit out of the airport.