Airline Club

They do

And the direction does matter in the simulation! For example you would see way more people going from other cities to a vacation hub airport than from that vacation hub airport to the outside world. In my design people always get round-trip ticket right now  but there's always a concept of departing airport (the passenger's home town) and destination airport

To see both direction, click on the passenger map button again it goes in rotation like this:

1. airport A -> airport B (all airline routes)
2. airport B -> airport A (all airline routes)
3. airport A -> airport B (only your airline routes)
4. airport B -> airport A (only your airline routes)

It's a bit complicated but it's definitely helpful to make your strategic planning! (keep in mind that passengers are much more likely to take connection flight offered by the same airline, so expanding your own local network would help your intercontinental routes ridership)