I don't think there is such a thing. Someone has to provide the platform so that players can find each other and compete against each other (a matchmaking system, so to speak). This would then also be the "owner" of the casino and would have to ensure a license, even if the games take place between the players of the platform.
A licence is some sort of trust among gamblers to play on that particular casino but it doesn't imply that they cannot be scam as some of the other casinos have turned out to be scam even after owning a licence but still new legit casinos feel it obligatory to own a licence to increase the reputation and trust of gamblers before entering the market.The decentralisation of casino also required some kind of trust among gamblers to run and verify each bets or any other kind of system to make it smooth.
Well, its an edge for a casino to have a license compared to those who dont have.
But it does not stop there, because they need to prove to gamblers that their casino can be trusted and not scam. Some casinos operate with license but their reputation was not good.
Having a license is good but building a reputation and gaining the trust of players takes time and thats vital for a casino to run exist for long period.