Obscurity is about hiding amongst the noise. I describe physical network separation, essentially air-gapping. Do we encrypt the signal between a car's brake pedal and the brakes? Why not? It is a very important network that could result in loss of life. We don't encrypt because the network is physically separate from attackers. If they are tapping into your car's internal wiring then they could kill you in any number of easier ways. A rocket tuned to only listen to emitters from specific locations need not encrypt because it is also physically separated from potential attackers. So in high-reliability systems if encryption/authentication only offers protection in hypothetical edge cases where the attackers is co-located with controllers, but presents any additional complications, it won't be employed.