I'm not saying they aren't good coaches, obviously they are but you have to temper that with the keys they've been handed. Robinson got the Ferrari in 2013 and all he had to do was keep it on the road. Sonny Bill, RTS, Tupou, Minichello, Pierce, Maloney, M. Jennings. He's had similar quality every year since. Bennett had the 90's model.
I think a better example would be Bellyache, who more than most does exactly what you said. He really has turned a lot of ordinary players into something special, builds teams around a couple of great players and fashions a Champion Team the measure of any Team of Champions. That takes a great coach.
Mary shouldn't even be mentioned in the same thread as those above.