The reason is as old as it is simple -- greed. Airlines can make more money selling 70 airplanes worth of tickets per hour than they could if they limited themselves to the 60 airplanes per hour that the runway can handle. In fairness to the airlines, it's not in their interest to limit themselves. It is easier to sell the tickets and blame the delays on the weather or the antiquated air traffic control system. Especially if the flying public doesn't understand runway capacity limits and therefore fails to notice that the antiquated air traffic control system is delivering more airplanes to the runways than the runways can handle.
A nice read.