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?
The reason for this is that a lot less people care about planes in Europe when you have an amazing high speed rail network that bankrupts Airlines all over the region.