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.
And there is no house in pool betting, poker or backgammon either. We still count all of them including betting exchanges (what has now been rebranded as prediction markets) are still normally regulated as gambling.