[kimos]

Airlines need to offer this as a single ticket option. (They might, I don’t know). Because the average traveller or the business traveller isn’t going to coordinate and book two systems, and the headache associated with one being delayed.

[MandieD]

Lufthansa does in Germany. They've cut their flights from a lot of the regional airports to Munich and Frankfurt (I think the government made them if ground transport time is lower than a certain amount), and replaced the former mostly with shuttle buses (Munich airport is not on a major rail line) and the latter with "AiRail" (Frankfurt is, and it's a high speed line) which makes regional hub train stations look like airports to the booking systems.