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.
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.
I've actually done that exact route once, and the train experience was much nicer, and took about the same amount of overall time.