Even if a cut isn't taken and there aren't other inefficiencies, any money tied up in long-term predictions is earning 0% instead of whatever the current risk-free rate of return is.
IBKR relentlessly advertises on the radio, so I’m aware that on their scheme you earn an interest like incentive coupon for every day you hold open the position.
Happy IBKR customer here. ForecastTrader has absolutely horrific liquidity outside of maybe 30-40 large contracts. The rest is all market makers that only offer 10-100 or so shares at each price point before bumping up a penny or two. No knock on IBKR as a whole, but you can't even effectively buy on most events or outcomes without slippage eating away your entire edge, and forget about real serious positions above a few grand entirely outside of those 30-40 big contracts.
It doesn't seem like it's strictly true that they don't charge trading fees.
From their docs, it looks like they charge fees to bet "takers" (as opposed to makers), but exclude the geopolitical and world-events markets where they don't charge fees.
I have to imagine that may be related to some of the blow-back towards prediction markets about profiting on topics like war & their potential for manipulation.
Given it sounds like the bot bets everywhere other than sports, many of those categories would likely have fees in this case.
Polymarket charges “taker” fees (people removing liquidity by matching listed orders) on most markets. Geopolitics markets are exempt. A portion of the collected fees then get redistributed to “makers” (people who provide liquidity by listing orders for others to match). Presumably the rest of these fees make up polymarket’s revenue.
Which is essentially also providing a platform for making the book for the other platform, on which 'bookie fees' are charged, but Polymarket itself only keeps a certain cut of it, for facilitating but not actually book-making.