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.
Yes, this is also true if you board the plane and your luggage doesn’t fit (for example, regional flights on a CRJ or ERJ). As long as it’s within the airline’s policy for carry-ons, they’ll gate check it for free.