Those are usually decentralized gambling sites with low liquidities which is why they don't really enforce KYC though there have been cases where they abused it such as Dexsport.io which is mired in controversies.
I don't know. I have never played in decentralized casinos, as it is kind of painful. You have to approve the transaction each time you place bets. I don't think real decentralized casinos have anything to do with the KYC system. I am not talking about these casinos. Some other unlicensed small casinos do not ask for KYC, and they operate anonymously. But since they are tiny casinos, they are not popular.
As for Duckdice, I have seen some scam accusation threads and drama here. But I think most of these are not valid. I saw a reputed member like holydarkness was harrased by these people who created the scam accusation thread against Duckdice.