I am almost certain that missile command and control systems are not connected to the internet. These would be on a completely separate, hardened military network. Critical electronic grid control systems should also be isolated on their own networks... and I suspect that most are.
Voter registration databases aren't in the same category, they don't pose an immediate life or death risk.
Anytime we design a networked system, we are balancing functionality and usability with security. We all love to do banking over the internet... which presents obvious security risks. We accept the risk because without them we would have to drive to bank buildings and hand our money over to human tellers.