Stupid, legacy calculus but calculus none the less. During the Cold war both sides recognized that a direct or near direct hit on a launch site could potentially negate any chance for retribution or follow up attacks in an otherwise limited/not all out scenario. So they counted the delivery mechanism they could identify, did some multiplication to account for misses and defenses and came up with a number. Then the other side noticed the build up, comes up with same general calculus and runs the same general math to decide how many they need. And this the goes back and forth for decades where each side is building up counters to their rivals counters. And the numbers we have now are actually a reduction from the peak of the Cold war build up.