The United States and Soviet Union spent huge sums on their conventional arsenals, what an opportunity for their militaries (especially for their navies) to boast their own conventional capabilities and to demand more spendings.
The American and Soviet leaderships were bon vivants rather than religious maniacs, why would they be eager to go to celestial paradise or hide in underground bunkers by triggering nuclear Armageddon when their adversaries had no intention of overrunning their heartlands and dispossessing them of their luxuries?