Ever wonder why sometimes it seems to take forever before the airplane door closes and leaves the gate? One of the possible reasons might have been because they were looking for that one missing passenger that never intended to show up. This would cause a delay to the inconvenience of all the other passengers. On flights with meals, that meal reserved for the no-show passenger would have gone to waste. As to double booking on different airlines, to compensate for passengers that make bookings and not show up for flights, airlines overbook to minimize empty seats.