The answer is simple: security. Attacker needs a way into the machine to control it. Granted, there are some obscure vectors of attack that don't need network c

The answer is simple: security. Attacker needs a way into the machine to control it. Granted, there are some obscure vectors of attack that don't need network connectivity (Stuxnet), however the game becomes much harder for the attacker when they have no direct connection to the target system. Many countries solve this by having a separate network for hospitals, but it is not the only way. In general, it is a trade-off between security and convenience. Yes, you can't send an e-mail without an Internet connection (well... not easily). But do you need to? From the computer that controls the MRI machine? Or is it just easier to say "we need Internet because updates"?