Because people don't show up for the flight. Arlines know a certain percentage of people will miss their connection, fly standby on an earlier flight, or resch

Because people don't show up for the flight. Arlines know a certain percentage of people will miss their connection, fly standby on an earlier flight, or reschedule. Margins are so unbelievable thin (airlines make more money selling food than seats) that they want every flight to go off full. There's nothing wrong with this practice. It's efficient. The problem was how United handled it. That said, the United flight wasn't oversold. They needed seats to transport the crew for a different flight.