You'd be surprised
Over a decade and a half ago I took a short flight in the LA basin and sat next to a water district employee. We ended up talking computers communications and he mentioned that a city's water system used microwave links for command and control. I asked what kind of encryption they used. He was confused, wanting to know why they should be encrypting the link, it was only doing water system stuff. I mentioned how it wouldn't be hard to override the HQ signal with a correctly oriented and stronger signal. He asked why they should be concerned. As this was before 9/11, I struggled to find a convincing reason and came up with, what if they override the pump controls and over pressurize the system?
He was shocked at the idea.
Seems I wasn't too far off the mark.