With a high enough voltage the air will ionized and the power will literally jump over many protection mechanisms. Also it can cause certain dialetrics (electrically isolating materials) to break as they all have a breaking point.

An extreme enough event can be way beyond even the biggest of tolerances of safety systems as there is some distance between where the outside end and the inside end are wired into the system and that distance is chosen with certain maximum voltages tolerances in mind which are finite and beyond those design voltages and as I said the air will just ionize becoming conductive and many isolators will just blow up.

So it makes sense that when a massive electromagnetic storm is inducing electric currents along tens or hundreds of miles long wires, the only guaranteed safe system is to not even have a cable from the grid coming into your house.