Most of the casinos do not ask for KYC and they have not made them mandatory. But if we see their terms and conditions, they write that in case of any suspicious activity, they can demand for KYC. I don't understand what could be the 'Suspicious' activity. If you won bog amount of money, most casino treat this as a suspicious activity. 
As long as you are losing a big amount, there is nothing suspicious about it but if you win big amounts, all of sudden your account becomes suspicious.
The casino can say anything related to the "Suspicious" activity but we are not sure about that but we are trying not to make them suspicious by always following their rules. As long as you are losing a big amount, there is nothing suspicious about it but if you win big amounts, all of sudden your account becomes suspicious.
The reputable and reliable casino will not ask too much if we win a big amount of money, especially if we do not do something "Suspicious" and allow us to send the money.
The casino will always monitor their member's activity to know that those gamblers who want to withdraw big money do not do anything suspicious.
So it is clear that you should not play on the casinos that do not have good reputations to avoid that problem.