Nobody is saying this they don't.
To play devils advoate, I wonder how many people each day get to fly specifically because the airlines overbook.
If I have a 150 seater, 6 don't show up, then that's 6 empty seats. Six people who had to find an alternative flight. Or you sell 156, 150 fly and accept that occasionally someone won't get to fly. Multiply that by say 1500 flights per day and you're talking a lot of people and a lot of money.