If you've only got carry-on baggage, then there's a small justification. If you're in an aisle seat, then you have to get up because the pax next to you is champing at the bit. If you've been sitting down for hours, you'll do anything that might pass for movement.
Mostly, though, people just want to get off. Any action towards that end, no matter how apparently counterproductive, will be taken in order to placate the Gods of Air Travel. It's the equivalent of wandering around the house opening and closing your fore and middle fingers while you're looking for the scissors.
R