Or would it delay passengers as they tried to open the lockers or argue with cabin crew about their need for said hand luggage?
Though that raises an interesting point, folk who rely on prescription medication are told to carry it in their hand luggage so it is accesable at all times, but are then told to leave it behind in the event of an emergency when some might just need it most or certainly afterwards.
Survived aircraft evacuation but died from lack of drugs afterwards!