Because all nuclear weapons expire within 30-40 years or so. At which they become dirty bombs or very dirty low yield nukes. To maintain a stock pile of nukes requires periodic replacement of the fissile material in the warhead. This is one of the reasons the DoE maintains super computers, so they can simulate the radioactive decay and the resulting explosion. Prior to this, this is why they did nuclear tests.  China was planning multiple invasions under the assumptions Bush, Obama, and Hillary would allow our stockpile to expire.