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.
Carry-on is not the same as personal item. My backpack does not fit under 90% of seats, and if it did, I would have nowhere to put my feet, thus the importance of finding overhead space.
I’ve not known any airlines that let you bring on hand luggage that’s too big to go under the chairs so consider yourself lucky to even have that!