True, and I'll not disagree but when the cost of a ticket per person is in excess of 300 dollars (for my flight to San Francisco), I'd hazard a guess that 99.99% of the people who buy tickets will be at the airport. I understand life circumstances can impact a person's plans to travel but generally I think that's the rare exception more then a rule.
You would be shocked at the number of people who don't show up for their flights. There's also the missed connections. Those that miss their connecting flights are rebooked by the airlines onto the next flight. However when bumped there are rules. They have to compensate you.