The seats are assigned. People have been standing in line for 15 minutes now. Why on earth would anyone want to stand there, when they could just sit and wait until the line clears?
I understand wanting to get off a plane ASAP, but boarding? You just end up sitting on the plane, waiting for everyone else to get on.
CGP Grey has a good video on a better way to board planes.
I know he acknowledges the capitalistic barrier to this solution but the other issue I see most often are folks simply not being there by boarding time. These methods all assume you would have everyone ready to board in their respective groups at once. I would imagine that throwing a few folks out of order in even the modified Steffen approach could bring down the theoretical time by quote a bit.
I like Bill Burrs stance as well