Not true, polymarket and kalshi have a lot of system bets, basically bets where the other side is the market, and further they take a cut from the money gambled.

There are other things as well, like they control which side wins, so they can leverage trade or even swing trade internally with a few bot accounts, not saying that they do but I don't see a ToC that says they can't.

Of course there is a house. You have a spread and probably a ton of other not so obvious fees. The house always wins by design, especially on polymarket.