They do it so they can relish in your look of disbelief.
Hey, I get them once in a while too. I also dislike the imbeciles who barge in before people have a chance to exit.
As an example of folks fighting to prevent a selfish apocalypse: My family was in a weird parking garage elevator last week trying to find the right floor to a hospital. Picking up on our confusion, two people guided us with righteous instructions.
Don’t worry, man. I’d hold the door for ya. But then I just might push all the buttons to see the look on your face.