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.
That said, Dutch governments seem determined to make Schiphol (Amsterdam airport) a hub, and from what I understand, they've succeeded in becoming a hub for not just Netherland, but also the UK, because Heathrow is unable to become a hub due to space constraints. Schiphol, on the other hand, has seen major expansion (a fifth take-off lane in the 1990s), and the Dutch government is considering turning a small airport in the vicinity (Lelystad) into an extension of Schiphol. Most people don't like that idea.
Frankfurt is also a major hub. There are plenty of hubs, but there isn't always space for them (like in London), and other concerns are often deemed more important than sacrificing all space to an airport.