One economic problem is that the express train to or from the airport (if there is one) is that a roundtrip ticket can quickly cost more than the roundtrip airf

One economic problem is that the express train to or from the airport (if there is one) is that a roundtrip ticket can quickly cost more than the roundtrip airfare these days. That is yet another barrier for disadvantaged people. When the airport is closer to the city they are often included in the normal public transit system and thus a lot more affordable.