My guess is that this is of no real benefit. Refueling, restocking, loading baggage, running maintenance checks, and other tasks need to happen before the airline can get the plane back into the air. Only extremely infrequently have I been on a plane that was able to roll back from the gate shortly after the last passenger boarded.

If the limiting factor here was passenger boarding times, airlines would have every incentive to improve the situation from where it is now. They have no incentive to speed up boarding to be any faster than the slowest of the rest of the preflight processes.