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?
I’ve taken jet bridges to board Embraer 175’s in the USA. In contrast, I’ve gone up stairs for an Airbus A321 in Europe.