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 heard of buses acting as trains (rail replacement service) but imagine being told your airplane is cancelled and you’ll get a replacement bus service
It is used a lot in Philadelphia to connect to small regional airports. American Airlines gets credit for serving the smaller airports without needing to dedicate planes and pilots to the route.