The current method of basically draining the plane from the front is probably about as efficient a way for people to exit as you are likely to be able to find. Multiple exits do speed the process but, they are pretty rare. If the plane is due for a short turnaround, the rear galley will be serviced while deplaning is occurring which takes that off of that table anyway. The process does not require explanation, conforms to societal norms of fairness and, except for a few outlying cases, just works.

The real reason why airlines don't focus on this? They have not been able to come up with a way to monetize it.