> In Frankfurt, I've actually had a bus take us from the terminal to the plane.

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.

Really? For me it was a first. At Schiphol, all planes park at the tube. I've been to other countries where they don't have a tube at all, and there we left the plane by a stair and walked across the tarmac to the (usually very small) arrivals building.

Well, most larger airports don't like you walking on the apron more than absolutely necessary, and if they don't have a jet bridge available they put you on a bus even if the plane is only a few yards away. But IIRC Frankfurt is the one where the bus actually goes out of the airport and down a public road?

Don't think so.

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.