It isn't just heat that damages materials. There are electrical insulators forming part of the transistors, and elsewhere. They are thin on the order of the process size, and are only engineered to withstand voltages slightly above the operation rating, just as a safety margin. If you exceed the tolerance voltage, the insulator will break down somewhere in one the millions of transistors. Once it breaks down, typically it no longer is a good insulator, and that will make your CPU inoperable.
On the scale of normal sized wires, the breakdown voltage of the insulation will be hundreds, or maybe thousands of volts. But for insulation as thin as used on present microchips, it is on the order of one volt.