And at Paris, which isn't. The difference is Amsterdam has one airport. Paris splits its international arrivals between Orly and Charles de Gaulle airports. The reason why has everything to do with the hub and spoke system airlines have developed all over the world. They gather passengers from many cities at one hub, then ship them off to their ultimate destination.
Local passengers traveling to and from the hub are the backbone of air traffic.
However, connecting passengers provide the volume to significantly increase the number of cities served with greater frequency. On average, about 60 percent of all passengers flying through a hub are connecting. That means the local passengers who fill the rest of the seats will be able to choose from far more flights. That sort of frequency is needed to help make a city a prime business center. The key to efficient air traffic comes with the creation of a system to serve both local and connecting passengers in the same place.