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.
People are posting a lot of maybe more rational reasons, but I think there’s another answer that’s more in line with just being a human. Airports suck, air travel, generally, sucks and the whole process is riddle with both intentional and also just unavoidable misery. Every time a new step in the sequence of unpleasant and boring steps that is air travel nears, we start to anticipate it and get anxious to move on to that next step in the process. It doesn’t make it faster, it likely only makes the misery arguably worse, but some times people just can’t help trying to mentally hasten things even if in reality nothing is hastened at all.