I found some of the SportBetting sites that claim to be decentralized and purely Player vs Player. I have tried some of them, and I could say that their system are not really good, even if you check their website now, it is not updated anymore. The last update are mostly from match last year. Moreover, their support are not that good either, so there are some, but not recommended.
A week team is behind such projects who don't even care to update the website once they lose faith in it. Such kind of working platform needs a 24/7 working support which would never go down and also the fairness should be clearly pure where both real players could play front-to-front. Such project could go much popular if the team behind it works hard. There are some common games like chess, carrom, etc which could also be made P2P with real winnings. I think a lot more peoples would then go for such platform and it would really stay much profitable for the industry as well as the players.