I've spent some time looking at group decision making for specific situations and there are clearly some examples where they work very well--and I think you hit

I've spent some time looking at group decision making for specific situations and there are clearly some examples where they work very well--and I think you hit on one of the factors. The individual guesses don't affect the outcome and aren't being used to hedge anything. I think there are some other factors as well but I'm not sure I've ever seen a particularly persuasive framework for when prediction markets can work and when they don't.