Because the purpose of them is to accurately predict events. People often get better at accurately modeling reality when money is on the line. Prediction markets are designed around the idea that you can best make money if your predictions match reality, so if you bet something that doesn't match reality, people have a direct financial incentive to model reality better and take your money. Some people are absurdly good at it. The net result, in theory, is a financial incentive to get accurate answers about the world.

"Partisan Bias in Factual Beliefs about Politics," Quarterly Journal of Political Science 10(4), 2015.

"You Cannot be Serious: The Impact of Accuracy Incentives on Partisan Bias in Reports of Economic Perceptions", https://ideas.repec.org/a/now/jlqjps/100.00014127.html