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.
Also, when you will likely be sitting for 2+ hours, I like to stretch my legs a bit before then. I don’t need to sit at my gate for an hour before going into a plane to sit for 3 more hours.