It would be trivial though to do the same thing for sports. Create the market, contracts on either side....

How is Kalshi paid today? They take a cut of each transaction, just like the bookmaker does. Neither cares who wins or loses

Turning it around, the bookmaker really isnt who you bet against, I get that technically you do. but in practice.... You bet against all the other bettors. Bookmakers offset bets with other bookmakers, etc if the balance is off.

In practice bookmakers arent trying to be on one side or the other in a bet. Some do, but they dont last long. They want the odds to make a 50/50 market in bets and they pocket the vig.

Oversimplified but that is the gist of it. Also, my knowledge of bookmaking is from my neighborhood and I assume the legal variety works the same way though.