And as mentioned ATC can contact a VFR pilot they may have inadvertently flown into controlled airspace by requesting an IFR aircraft that is already on an ATC frequency to call on the 121.5 guard frequency. That's the reason the FAA has this 121.5 requirement for
So currently if ATC needs to speak to an aircraft they call the aircraft on centre, hopefully said aircraft responds, ATC passes on whatever they need to, and then the aircraft responds and everyone goes on thier way.
But you propose to have ATC calling some passing airliner, the airliner responding, ATC passing the message to them, airliner responding, airliner calling the VFR aircraft on 121.5, VFR aircraft responding, airliner passing on message, VFR aircraft responding, and then the airliner going back to the original frequency and letting ATC know the message has been passed on? Who cares if that's what they do in good ol' USA, that doesn't sound overly efficient. A lot of wasteful radio transmissions there.