If everyone leaves room to move over, stopped traffic takes up the entire road anyway and the ambulance can never get through.

You think multiple lanes of traffic can all fit in the shoulder, which is generally smaller than one of the lanes? It's not physically possible--and that's where there is a shoulder at all.

And still not helpful at all for cars stopped at a red light who have nowhere to go. Cars can't just move sideways, they always have to go forwards unless they're moving backwards.