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.
Two reasons:
I always take window seats, and don’t really want to hop over someone to get to my seat.
And it also means I don’t have to fight for overhead space for my carry-on.