You can't sell things you don't have

yes, but selling excess tickets is because not everyone shows up. We all know that one person that constantly shows up late. If you're wondering why after check-in has closed, why not put through, it's because airlines have to lodge a passenger manifest with the FAA, changing this after it's been lodged can costs tens of thousands for the airline in penalty fees. So it's not in their interest to allow a passenger to check in. This is why it's advised to be 2-3 hours at the airport before your flight is due to leave.

I used to work at hotels, and a pretty common idea was to oversell 10% of your arrivals. So if we had 100 check-ins, we'd oversell by 10. We would generally have more than that in no-shows, so it wasn't an issue, but sometimes we had to "walk" guests, meaning that we would give somebody one night free at another hotel if we didn't have a room for them.

I've read a few different articles retaining the same information aswell as a few other people have told me this it's not just united the only reason that situation stood out was because of how the situation was handled. it is common to have to people removed from flights. As far as I am aware they usually offer some sort of compensation if necessary. ¯\_(ツ)_/¯

Stand by tickets were a much better system. If there's a flight with no show's, the first people who bought a stand by ticket get the seats. If the flight is full, you wait until there's a no show on a flight to your destination.