In theory, an insider willing to take any positive-expectation bet would drive the market price arbitrary close to the correct answer, but in practice they might be under NDA and want to avoid getting caught, or they limit their bet amount for other reasons, so the market ends up dominated by gamblers placing uninformed bets.
Indeed, I'm just surprised.
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.
You say in practice, NDAs. What we've seen so far: classified military ops, people with access to Presidential speeches...