Radar sweeps, it takes time to get back, it takes time to relay information and it takes time to respond. When you have aircraft moving at hundreds of miles an hour crossing in close proximity the trajectory can change and result in impact before radar can gather data, assess it, make a prediction and relay that information.
You might be interested to know that the video feed (the marketing one at least, not the one for engineers) for the recent boom supersonic flight was just a phone and a starlink in one of the planes following. Things can be done.
Why technology still does not track every airplane at the airport and flag runway incursions.
Why there still are not cameras constantly recording flight ops at the airport. Why are we relying on accidental dash cam recordings?
Why the cockpit voice recorder still does not record video.