This absolute tripe of providing continual traffic alerts to aircraft 3nm apart (5km) when there's not a hope in hell of them hitting is ridiculous.
It is a joke. (And, like you, I'm not criticising individual controllers for providing this 'service'.)
The blips on the radar screen might look close, but out there in the real world ...
I have related, before, a story about a helpful controller who alerted me and another aircraft as to each of our relative positions on approximately the same track, with me slowly catching up on the other. Severe CAVOK. About 5 minutes later I spot a tiny speck in the distance, to whom I eventually caught up and waved as I passed about half a nautical mile abeam and 500' above. (The other aircraft was IFR.)
The joke? The gliders we whizzed past in closer proximity were, of course, not on the controller's radar screen and therefore not notified as traffic, yet we managed not to collide.