Earlier quoted context omitted.
Or they're observed over military facilities because that's the only location with a rigorous surveillance system.
Or because they're sensor artifacts on military systems and those systems tend to be flown over military facilities.
https://en.wikipedia.org/wiki/Clutter_(radar)
Radar and infrared aren't infallible. The story of the pilot who saw something with his own peepers has my attention but I don't trust what they've only "seen" on instruments.