Many major airports in the US aren't set up for remote stand deplaning and busing back to the terminal, or is done only infrequently (and at an additional cost to the airline). The US3 all have banked hubs which concentrates arrivals and departures. Consequently gate space becomes limited and if there's an issue like a late departing flight still on a gate for which an a/c has arrived at least with AA that a/c is forced to wait until that previous a/c departs.