The short answer is because the common method is the slowest method. The longer answer is probably because… money: a lot of people will pay more for a first class ticket just to avoid the horrors of boarding. Vox has a pretty in-depth article on the five different methods of boarding a plane:
- Back-to-front, a.k.a. the Standard: people in the rear of the plane board first. This is what most airlines do, and it’s been proven to be by far the slowest method of boarding a plane. Why? Because a lot of the time someone already sitting on the aisle has to get up to let someone sitting by the window through, and people have to access the same overhead spaces, so everyone ends up waiting and waiting. It takes 173 people 25 minutes to board a plane in this way.
- Assigned Random: no order; people just line up and go to their assigned seats. Because they’re scattered all over the plane, it actually takes less time than the back-to-front method. So here’s a situation where engineering has actually made things worse than doing nothing at all. This takes 17 minutes.
- Outside-In: like back-to-front, but with people by the windows boarding first, then middle seats, then aisles. No one has to get up to let anyone else in, so congestion is much decreased. United does this now, and makes exceptions for groups traveling together, so they all board at once. Down to 15 minutes.
- Unassigned Random, a.k.a. the Southwest: just like the bus, in which people get on and sit somewhere. This is faster than the Assigned Random because people will just sit somewhere rather than wait in line for their assigned seat. Few really like this method though, due to the added stress of “will I get a good seat?”. It makes us feel much better knowing ahead of time that we’re stuck in the middle. 14 minutes.
- Optimized Outside-In, a.k.a. the Steffen: just like the Outside-In method, except only one side of the plane is sent in at a time, and rows are skipped to keep people from using the same overhead space. The method is named after a physicist studying this very important problem. Experiments done with this method were for 72 people instead of 173, but extrapolating from the proportions, you get about 13 minutes to fill the 173-person plane. That’s almost twice as fast as the standard method. Here’s a video demonstrating its awesomeness:
The Vox article has simulation videos of all of the boarding methods and more details on them.
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From Vox, via… Gawker, maybe?