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.
You can bend your legs under the chair? Obviously I’m exaggerating but I don’t think it really takes away that much of the tiny amount of leg room you get anyway!
With the backpack I have, I do lose a good part of my leg space from having my backpack underneath the seat in front of me. That’s why I sometimes pull my backpack out and then set it down in front of me, but not underneath the seat in front of me; this lets me stretch/move my legs more than before.