Arriving into CLT tonight from CVG enroute to MCO, our inbound arrived and was delayed 10 or so minutes. Made our way to B from E gates, got to E8 at 7:52 for US747 that was scheduled to depart at 8:05 and the door was shut. I was the third to arrive, only to be followed by 23 more passengers for a grand total of 26. As the GA told us that the tower wouldn't let them hold the flight, she proceeded to then tell us that they can't open the plane door once it is closed as it becomes pressured - and opening the door after it is closed will cause all of the air come out of the plane and the plane won't be able to stay in the air once it takes off. So as the GA manually rebooks the passengers, we watch as US747 waits at the gate for another 25 minutes and finally pushes back at 828. Now you know the real reason why plane doors can't be reopened once they are closed.