Emirates has that policy with most (if not all) lounges outside of Dubai. They just open 3-4 hours prior flights. I would also presume that it’s around waste and staff cost
As do many carriers outside their hubs. For example - SQ in MEL close their lounge in between flights such that it is only open 3 hrs before flights depart (unless another flight is within that 3 hrs in which case it will be open 3 hrs before the earlier flight).
The main exception is where an operator contracts to other carriers (eg NZ lounge in MEL stays open until TG departs - Long after after the last NZ flight departs). IIRC EK generally doesn’t contract their lounge to other carriers.