if you had any idea how many pax regularly get up before the a/c comes to a complete standstill at the gate then you would understand why that announcement is made.
Overheard on a LOCO, "As we have a limited amount of time for our turnaround we would appreciate the help of some passengers in cleaning the cabin. If you would like to volunteer please leave your seat before the aircraft has come to a complete standstill and the Captain has switched off the seatbelt sign."