There’s no “house” or “book” aspect to Kalshi. They are nothing more than contracts that are bought and sold between individuals.
Have you heard of Betfair? A UK company which has done "prediction markets" since 2000. In the Europe it is called a betting exchange and regulated like sports betting. And betting pools is another form of betting which is way older than that and there are no house in them either.
And by your argument poker or backgammon would not be gambling either.
Lots of types of contracts bought and sold between individuals are prohibited by law.