Prediction markets that trade in play-money rather than real money seem like a great idea to me: they avoid all the legal issues of real prediction markets, they don't seem to be much less accurate, and it might be fun to participate in them. But I can't find any active ones online.
The only ones I can see are ideosphere.com and alphacast.cultivateforecasts.com, but there's almost no activity in either. Metaculus is very active but isn't a prediction market. I've also heard of a few like NewsFutures which have completely died. Are there any active ones that I'm not aware of? If not, why do all play-money prediction markets die?