Flying in Europe, it is about a 50/50 chance as to whether you get on the plane from a jet bridge or you take a bus to the plane parked on the tarmac. In contrast, most US airports have jet bridges, even when the plane is small unless it is a very small airport.
Why?
This is a great question and I wanna know too. The first time I set foot on a tarmac was probably when I was 22, and I had flown dozens of times before then.