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.
From the airline’s point of view, having the next person right there ready to get in their seat is preferable to having everyone come up one at a time. This is why they have boarding groups. You usually see between 3 and 5 boarding groups because it’s a reasonable number between 1 and N (N being the total number of passengers). I’m curious how ~10 boarding groups would fare, but presumably there’s a reason airlines don’t do that.