Well, no, it isn't always true that everyone in the aisle is already ready to get out. Sometimes they have been unable to retrieve all their overhead luggage because of other people's bags, sometimes they want to help their fellow passenger, or their child, to get up and ready, and so on. Sometimes they're just plain disorganised.

But I think the bottom line really is that getting out of a plane can't always be done in any strict order, and everyone has to exercise patience and a little "give and take".