this is precisely what a friend of mine told me 30 years ago (this dude is like the Edward Snowden of Britain; he still lives under a level of secrecy to the point I had assumed he had passed on due to his age but tracked him down via an electronics hobby forum).
He explained it in very simple terms :
“if you put a signal into the radio airwaves, everyone within range can receive it – whether or not you wanted them to. If instead you send the signal along a wire; unless you can see both ends of the cable and inspect it for its entire length, it may be tapped and intercepted”. Even if you encrypt it; the header (metadata) must be in clear or the network cannot route it; and the signal will always travel at the speed of light so its location can always be discovered”.
This advice is still true today.
