And for short trips, most time is wasted by having to check in two hours in advance. High speed trains are better for short trips in my opinion. But as far as I understand, the point of a hub is to concentrate traffic for the long trips. Those 20 minutes suddenly don't matter all that much anymore.
This is very common even in smaller airports when an airline doesn't want to pay for the more expensive placement at the tube, as far as I understand. Also some airports have a road for airport vehicles just between the building and plane parking position, and don't allow people to walk to the plane for security reasons.
Feels like most of my flights start with a bus to the plane.
Though, you have a high change of crossing over the Autobahn with your plane if you arrive with a 2 engine model (the HEAVYs cannot use the northern runway, as the runway is too short for them)
And some planes are parked rather remotely, where the bus trip might take around 10-15 minutes and is not that much different from the official guided bus tour of the airport.