Kalshi and Polymarket take a cut and is making the markets as well. Yes you can create a market and fund it yourself, but almost no one ever does this. People have been trying to make true prediction exchange markets like this 3 decades, its basically impossible without professional market makers.

Its largely impossible to fund "organic decentralized" prediction markets because the vast majority of money that goes into them is "dumb money" and there will always be sharp money that takes their cut, and has substantially more funds. Betting markets are zero sum, so the smarter participants is always going to absorb the entire bankroll of the dumber participants over time..

The closest thing to what you describe were BTC dice games, which is kind decentralized, but prediction markets are impossible, some smart guy is always going to be there to make a more accurate fair on a market and eat all the liquidity from the little guys.