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?
Most American airports are space constrained, including many of the oldest ones in the country. The old airports in the US generally use jet bridges even when they are constrained by space.