One part defense contractors profit obviously they would rather build more than less. One part arms race. If you keep building better and better, you also have more One part first strike/madd. You gotta be able to have enough nukes left after a surprise attack to turn the enemy into a wasteland. Alternatively, you want to be able to take out all the enemies nukes at once, and have them by the balls. So the only way to stay in the same place is to run forward as fast as possible