Because in real life weapons need maintanance. You can't just manufacture warheads and then leave them in some cave... perhaps in computer games it's works like that, but in reality things are extremely complicated.
Huge industry is needed to maintain warheads and you can't hide that sort of activity... modern nuclear warhead is an weapon system itself what has batteries, explosives, software, safety systems, and everything else crammed into small package.
All that requires skilled workers who have to live near their work places, and you can't hide human activity related to that sort of operations, etc.