I think scale is the answer to that. These systems are being scanned for day and night for vulnerabilities from attackers and once they have infiltrated can lie

I think scale is the answer to that. These systems are being scanned for day and night for vulnerabilities from attackers and once they have infiltrated can lie in wait to do something whenever they want. A foreign actor can do that at massive scale undetected (both when successful and when not). Same isn't true for physical security.