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?
Until recently, La Guardia in NY, and Philly airports had pretty regular busses to commuter jets (Embrairs, Canadians, Bombardier, etc). Philly still busses between terminals. I think DCA does as well. Not sure about others. I used to go through LGA, PHL, & DCA pretty often. The CLT (have to include “the”) also has a lot of outdoor boarding from terminal E (the commuter terminal).
So, in the US, you see busses more in larger airports that are more land-constrained for terminal space. Beyond this, you’re getting into small regional airports when you see more outdoor boarding. As to why, it’s probably just in response to expectations of US air passengers.
Buses between terminals is a lot more common and I’ve even seen buses acting as airplanes in the USA.
I can see some US airports using bussing, but it feels like the practice is far more commonplace in the EU than the US. It is also odd given than EU airports appear to be newer on average to US airports.
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
Them Duke boys are at it again