So that they don't crash into them.
And military would have encrypted insider communication between eachother
It may be naive question cuz I never operated such a thing
Then you have to rely on a human element, which is a horrible idea in a rapidly changing environment.
Imagine someone hits the base and takes out its comms. You no longer have any idea where anyone is.
And yes, there are people who have cameras pointed at the sky all over the country, and with software that says "I see a plane, and I don't see no ADS-B".
Combine https://www.tomshardware.com/how-to/raspberry-pi-airplane-tr... with https://medium.com/swlh/automated-meteor-aircraft-satellite-... and you're golden.