If an insider with large amounts of capital makes a big trade, they also end up discouraging other trades. Once you see a huge position taken, LPs are going to scale back their liquidity in other positions to manage risk that the insider is going to stomp them. Any trader monitoring position sizes is going to probably scale back their trading. All of this contributes to less trading and less commission on these markets.

Sports betting is so profitable for prediction markets because they're mostly unsophisticated retail flow making lots and lots of trades, giving the platforms commission. If an insider just pushes market prices in their direction the platforms are going to lose on volume.