I don't get this. How are people "betting" on something that is "known information" for other people? What's the point? I get betting on stuff that no one can know, but who's taking "the other side" on these bets? Why would you put money into something knowing that there are people who already have the absolute answer?
Insiders have an edge and can in theory shape the overall distribution of bets such that the real world outcomes are priced in better.
Maybe the people who could take advantage of these markets are just not sufficiently aware of their existence.
I predict that in 5 years "who wins Survivor" will be completely dead though, because the markets will have become common knowledge and there'll be enough people willing to violate their NDAs/whatever for the money.
In prediction markets, imitation is the best policy