But they don’t work without someone making the argument that some non-insiders should bet on them right? Because the insiders aren’t going to bother moving the market for us if there’s nobody to take money off.

The traditional way to square that circle is to say that someone who's interested in the answer should subsidize the market. Essentially they provide liquidity to the market which essentially pays people with accurate information to bet. Some work has been done to figure our effective ways of doing that. In practice, it seems like gamblers often provide enough liquidity that this is not needed