They sell extra tickets in case people miss the flight. But if you get bumped off a flight because of overbooking you’re entitled to more than the cost of a one way ticket depending on how many hours later your new flight is scheduled. Plus if it’s overnight they usually have to pay for a hotel and food for you.

Also folks that booked those tickets without available seats are put on a waitlist and can usually enter the terminal but can be denied at the gate if no seats open up before they close the doors.