I think it's more about bragging and joining an exclusive club (example: I have this opinion, and I have a pile of money sitting around that I can dedicate to promoting it amongst a small group of other people I want to impress). The stated purpose of holding people accountable, I have never bought.

People who are really working on the ideas discussed (as opposed to just talking about them and positioning themselves as "futurists") are accountable in a much more profound way, in that they build their entire lives around working toward some goal, with all the risk and opportunity cost that entails. The long bets participants are just playing around, not real players. They may have a little skin in the game, but it's faux skin.