COs know that if they just tell all the elders, "You guys are doing a great job. Keep up the good work," they won't be COs very long.
The GB is always demanding more of JWs -- more field service hours, more members, better attendance at meetings, more members pioneering, more people attending the Memorial, more sacrificial donations of money, and so on.
This puts a lot of pressure on COs to show better results, and COs pass that on to the elders.