Re: See The Film Wargames
Humans *are* the weak link in the chain, and the US certainly knows it and plans around it.
As in, no you don't need enough weapons to nuke the entire world 3x over, but they know very well that in all probability something like half of their crews will never follow the order to launch. The whole point is that the people with the keys have to go through all sorts of tests to prove they're sane enough to be in command of weapons that can bring about Armageddon - and yet, no sane person could ever follow the order to launch.
I'm sure it's been considered to replace the human element many times, but cooler heads have prevailed.