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.
This is why I do. I typically just take a small bag for under the seat and a small carry on suitcase. Some people have so much shit, usually after a long flight the last thing you want is to be the last person off the plane because your carry on is 8 rows behind you.