In recent news (cn: autoplay), a United Airlines passenger was unwillingly dragged off a plane to make seats for employees. The incident was partially blamed on the common practice of airlines to deliberately overbook flights in order to make up for all the no-show passengers.
While I won’t discuss the particulars of this incident, I am willing to do something that most journalists are not: read relevant academic literature. I just picked one paper that appeared to have a sufficiently broad perspective:
The basic problem is that many people who buy airplane tickets don’t show up. In 1961, the Civil Aeronautics Board (CAB) reported that 1 out of 11 ticket sales were no-shows. These numbers are about the same today, with 7-8% no-shows. The airlines could create wait lists to fill the empty seats, but it would be impossible to contact the wait-listed customers in a timely fashion unless they were already present at the gate.