Why Is So Much Sensitive Information Placed on Internet Connected Machines?

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.

One of my current projects is for a government contractor with very high security requirements. It is a pain in the ass for everyone involved. I had to have extra training to ensure that I won't fall for phishing schemes or download viruses. And each time I transfer files, I have to go through two levels of security with 2 factor authentication.

The short answer is convenience. The more interconnected things are the more convenient they are to work with. The long answer on security is that the level of security you apply should be proportionally balanced against the value or risk of the thing you are trying to protect. More valuable or risky data should be harder to access.

That being said, I would note that many many things on networks (not just the Internet) are very much NOT balanced proportionally with their value/risk.