Glider pilots also have the advantage of knowing what a glider can and can't do, and what the generally agreed procedures are for gliders either in a thermal or in the circuit. They instinctively know where to look for the best place to spot other gliders. Plus, gliders do circle around a lot, making for a better visual target.
In the circuit, powered aircraft generally have the same advantage. We know more or less where other aircraft should and should not be, in what direction and at what altitude they should be flying. (Assuming they behave themselves properly.) So we know where to focus our scan.
But in cruise, powered aircraft may fly at all altitudes, in any position relative to convective clouds, in any direction, and generally in straight lines. Makes it very hard to spot aircraft in cruise.
I would not be surprised if gliders, because of this, can spot other gliders miles away, but are regularly surprised by a powered aircraft which cuts all of a sudden through their thermal, oblivous, seemingly, to the glider traffic. And the other way around too.
Oh, and gliders have far less blinkenlichten in the cockpit to play with and get distracted by.