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.
Southwest assigns you a position in line. You can pay for a better one. Their passengers don’t spend a lot of time waiting in ad-hoc lines before their group is called like other airlines.