Does this apply only to superpowers over a certain level of military strength? Otherwise one could wonder why Mexico or Canada which have no comparable military power to the US hasn't been invaded yet. I agree in principle that mutual assure destruction can be indeed a valid deterrent, though I hardly believe there aren't others in a civilized world.
To me also there's not that much difference between mutual assured destruction and severe damage. Let's say the US has 100 nukes and Russia only 20 (random numbers just to make an example), all of same power, and let's assume that most of them will be neutralized before hitting the targets so that in case of mutual attack Russia would be hit by say 30 nukes and the US by just 4. History 100 years from now would surely regard it as an epic victory for the US, but today anyone ruling over there would have to explain their citizens how on earth they couldn't take a step back and spare millions of lives.
It's probably much simpler than that. The US, Russia, China, North Korea, etc. have zero intentions to go at war with each other, but is in everyone's interest to keep fueling the weapons industry, so they keep playing the plot: citizens are hooked using nationalist arguments to make them supporting their rulers etc. Weapons keep being amassed until the day they become obsolete and can be sold to less rich countries for their smaller wars, new more advanced weapons take their spot and the business continues. It's a win-win scenario for everyone but common people.