Sure, you can get the same security by isolating modern machines from a network and loading code using USB's or CD's and DVD's, but why fix something that's not broken? These systems only have one job, and they were a significant investment when they were installed, and the still do their job pretty effectively. The US therefore has little to no incentive to upgrade the systems already in place.
The other thing worth mentioning is the simplicity of these systems. Older hardware is suprisingly easy to service, modify and alter purely because they're so primitive. Sure all those discreet components might be less reliable, but when something does go wrong, it's often a case of a bit of solder and a new component, instead of buying and installing a whole new SoC or daughterboard.