About CDG, if you fly in a turboprop or regional jet, you take a bus and walk.
for bigger planes, it depends if there is an airbrige available at the time the flight arrives, and if the airline wants to pay for it. (the airline will pay less for the bus than forthe airbridge)
another reason is that if an aircraft has to stay at the gate for 4 or more hours, the airline prefers a remote parking , as it will be cheaper than using the airbridge for the arrival, and then towing the plane to a remote parking, and the coming back at the gate.
I think it is the same in most of the European airports.
When Linate was the main airport in Milano, you had to take a bus.
In STN, the fastest growing London airport, most of the airlines are low cost (Buzz, Ryanair, Go)
Space is not a problem. and the aircraft are just parked in front of the airbridge. But those airlines do not want to pay for it. Then, you walk...
Ecoute les orgues, Elles jouent pour toi...C'est le requiem pour un con