Could never understand why someone could not invent a simple transmitter receiver that could send a signal say 5 miles range.
It could say transmit a signal for 2-3 seconds then listen out for 2-3 seconds.
A simple beep in the cockpit would warn of the presence of other aircraft.
Make it cheap and make it mandatory, after all we all have to have medicals, and cough up the cash to do, so surely a device could be made for not much more.