Correct, but there is a direct correlation between the size of the market, and the power of the people determining the outcome.

Then there is also the fact that the power of the people determining the outcome is inversely proportional to their care about betting markets.

Put this together and you get "The larger the size of the market, the less the people who can single handily swing it care about doing so".

If you are someone who can command hundreds of thousands of people to bet millions for or against you, you almost certainly lose more than you would gain by gaming it.

Mind you the market also naturally prices in this risk of the one person going rogue and taking them winnings for themselves. You will never find a market for "WarmWash will post nothing for 3 days straight on HN" because no one will take the other side of it.