Here is one hypothesis:

Because consumers want and value assigned seats. Overhead bin space is scarce as well, and airlines allocate that to their best customers through early boarding. In other words, there are competing business objectives. It turns out that the data on boarding processes is mixed. You can do simulations of faster boarding but it turns out that

* What’s fastest changes, you don’t actually get consistent results across airlines and over time

* There are switching costs, in terms of training agents and disrupting passenger routines

Do you agree with this analysis, or are airline managers just plain stupid? More here. (hat tip (yet again) to Tyler Cowen, Master of the Internet)