The reason we have so much stockpiled is because of minimum production rates. Any production facility needs to keep the lines moving. If you stop the lines, the price per unit skyrockets. To keep that from happening with really important stuff (like ammo), the government has a minimum that they have the ATKs produce per month. This builds up, because even if they have surplus, the government charges the services to fire those weapons. During the late 90s we didn't exactly go through a lot of tank rounds, or other ground vehicles (like MLRS or Bradleys).

That means it is a surplus. Know what is funny (since it is your tax dollars at work)? It is cheaper to have this surplus than to stop the lines and start them again.