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.
The problem is that you never know, which type of activities may be considered as Suspicious by a particular casino. Big wins I think are always treated by any casino as the case for additional check.
The shady casino will use many ways to trick their members because their purpose is to freeze the account so they can possess their funds without having the intention to refund the balance.
It already happens to some people and even if they complain to the site, the site will easily ignore them.
So only research will help us avoid this thing so we can play gambling without worry.