A common point in many of the recent accidents was that ATC was unaware of them. The reasons for this vary, but another sensing tool might be a remedy.
If they are flying in uncontrolled Class G airspace, then ATControl don't need to be aware of them and actually don't want to have to be aware of them. Awareness -> control -> responsibility -> too much workload.
In GA, if you see another aircraft you normally alter course to get of each other's way. If you are following a common feature (motorway, railway) and you don't see the aircraft doing the same in the other direction, then hopefully you will both have the feature on your left.
In gliding, OTOH, if you see another aircraft circling and going up, you think "oooh, lets go and play together". In extreme cases in competitions you can have situations that I rather doubt ATC want to be involved with, e.g. ...
Sensible? No. Fun? Looks it!
Please note I've previously made a point about top glider pilots pushing the envelope, and what can happen.