Why do over-currents destroy IC's?
As current increases, I2R losses increase really fast, you get to a point that the IC will hit its temperature limit even if the heatsink it sitting at 25'C, the internal thermal resistance puts an upper limit on power dissipation. Push the chip hard enough and you could even melt the bond wires (this is what "package limited" means in some mosfet datasheets). At the heart of it, the main limiter is temperature, just like with an electric motor, if you can keep it cool (and you can keep the switches fully enhanced), there's not really that much of a limit. Semiconductors can handle some pretty crazy current densities. (there are physical limitations like not having enough free electrons to carry the current or electromigration where the force of all those electrons literally shreds the metal conductors like an atomic sandblaster, but with power electronics, 99.9% of the time, it'll explode due to instantaneous superheating first).

However, in processors with nanometer scale transistors, electromigration is much more of an issue, the wires are so small that it doesn't take much for the force of the electrons bouncing into metal atoms (definition of resistance) for the metal to start moving in the direction of the current, this can eventually lead to the wire breaking, but that's not an instantaneous process, like corrosion, it takes time. Instantly killing a chip with over current is almost always going to be a thermal issue.