Like most complicated mechanical things, nuclear weapons have a limited shelf life and require maintenance and replacement of components and materials that age out. Thus, after enough time, nuclear weapons that are not maintained are no longer operable. Granted, the baseball-sized chunk of fissile material in a backpack nuke would remain dangerous, but at least a nuclear detonation would no longer be possible even if the armament code can be broken or overridden.
And you don't know which one it is.
And you don't know the exact voltage it should be.
And it discharges to an unacceptable voltage after a relatively short time.
And the whole thing is a paperweight without the right array of voltages.