Battleships: a ridiculous but awesome idea 15 years ago
I think missile silos are stationary is pretty easy to understand. They're easier to guard and if you look at when they were built rather than their current functional use (its debatable if there is one when looking at SLBM and B-52/B-2 dropped nuclear weapons). In the 1950s, when ICBM projects began, the threat was not thermonuclear weapons, but fission weapons. It was thought that the silos might survive a direct/indirect attack on a facility.
After the advent of the hydrogen bomb, no harden target can really be called safe. Coupled with modern military GPS and you have a super-precise, super-powerful weapon.