Because the airline needs to know how many passengers will be on the flight and they need to have the names of the passengers. With roughly 10 percent no-shows (passenger buys cheap ticket and does not fly) you would never know if you have got all passengers on board! And I can tell you - itīs trouble every day to sort out missing web-check-in passengers.
Again, maybe I'm missing something obvious - but is there any reason why the computer system that identifies the barcode on the boarding pass at airport security couldn't be tied into this? I'd have thought this would be by far the easiest way to see who has actually turned up for the flight. Just wondering...