What if they crack the safe?
Once again, that is the balance between physical and electronic security? They would still need to get into the computer room and know how to operate the box. Also the security compromise would be pretty obvious.

One last thing nihil - weren't there some missiles whose launch codes were 0000 or something?
Yes, actually this is not surprising or scary. You are talking about battlefield/tactical weapons and those dropped from aircraft. You are already at defcon 1 in that situation.