If they let people who stand first, deplane first, then there'd be a free-for all in the aisle and many people would get hurt. Not to mention the fights that would break out as folks rush up the aisle in a full on sprint, banging passengers in the backs of the head with their carryons in their attempt to be 'first'. It's kind of like that jerk who blows past you through traffic at 20 mph above the speed limit only to be stopped at the same red light that you eventually ease up on while he's waiting. Then you get to repeat the exact same experience making it to the next red light.

Patience is a virtue.