To an airline, a flight that leaves with an empty seat is basically throwing money away; that empty seat doesn't mean you can take less fuel, fewer flight crew members, or wear out the airplane less. Therefore, the empty seat saves the airline no money.
Furthermore, people often miss their flights.
Overbooking is one way to deal with these two assumptions. Assume a certain number of people will miss the flight, and sell some seats twice. Then you don't fly with money-eating empty seats, and you can lower the ticket price a bit, because you don't have to charge customers the cost of those statistically-likely empty seats.
There are other solutions. Don't offer any refunds or reaccommodation to no-shows. The seat was empty, but someone paid for it, so who cares? But customers probably don't like that. The might even like it less than overbooking. I don't know, I don't run an airline. There are options, but overbooking exists for a reason.
For some numbers, a 777-200 carries 313 passengers, weighs 135k kg, carries up to 100k kg of fuel, and has a max takeoff weight of about 250k kg. 313 passengers and luggage, averaging 100kg, weigh just 31.3k kg. Your ticket therefore pays for your weight, plus 5x your weight in airplane structure, plus (up to) 3x your weight in fuel.
If you don't show up, they're still carrying 8x your weight anyways, so saving 200kg isn't that important.
(PS: apologies for the 'thousands of kg' units. I thought it was more readable than Mg or fully written out numbers.)
Just write tonnes (= metric tons)?
What about if you are flying long haul and linking up to a domestic flight on another airline? "Oh, we can't help you because your previous flight was delayed due to weather/mechanical/congestion.you should have flown on one carrier even if the price was 2x"?
Airport congestion is a big issue in the US, as is weather related delays.
I was under the impression that the fuel load for each flight is calculated based on a formula that averages the weight for each passenger and their expected luggage. Fewer passengers than expected = more fuel left over after the flight.
Are you sure of that?
I would think that airline profitability is largely dictated by competitive forces, rather than the particulars of in which practices the overall industry is permitted to engage.
(Note that I'm assuming a world where all airlines, or no airlines, overbook. If only some of them overbook, then I agree the practice could affect relative profitability of those who do / don't overbook.)