That isn't very convincing, as the stock market itself is largely a prediction market. People buy stock to bet on future success, whether that manifest in the form of stock price increases, splits, and/or dividends. It's merely a much more narrowly-focused prediction market.

For that very reason, insider knowledge, and especially the ability to influence future outcomes, become the subject of heavy regulation. And, the lack of such regulation for congressional members is also why their net worth tends to skyrocket once entering office.