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.
Jet bridges are also an access control system. It’s harder to wander off to another part of the tarmac if you can’t get to it in the first place.
It’s also probably cheaper to guard too, since locked doors and alarms don’t require healthcare, and you can push that duty off to flight attendants and gate employees.