Not all aircraft have Mode S EHS transponders, sleeve of wizard.
If the QNH is 1013 and I set 1033 in my steam driven altimeter then fly at 6,100' indicated and report maintaining 5,500', what is my steam driven 'blind' encoder squawking as my altitude and how would ATC know what QNH I've set on my altimeter?
And if the set QNH is reported on aircraft fitted with Mode S EHS, why TF does ATC require readback of the ATIS QNH after the crew of the aircraft fitted with that gizmo have reported receipt of the ATIS information code which includes that QNH? Surely ATC will know if the correct QNH has been set, via information given to ATC via the gizmo.