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?
Maybe something to do with ADA (Americans with Disabilities Act) compliance? Europe tolerates a lot less accessibility by necessity since they have so many older buildings, so maybe that translates over into having more tolerance for stairs when boarding planes?
“Europe” has different laws in different countries. The UK is incredibly keen on accessibility, but Germany is ablist as hell, for instance.