The casino does not ask KYC at the time of registration is to avoid new users from running to another casino because it is already faced with choices that are not liked because KYC is not something that players like even though this is no longer avoided.
And asking KYC when withdrawal of course there is a reason usually when the casino detects suspicious activities, but the reason is still unpleasant for the players and I have experienced this sometimes, and when asking the reason it was said because it detected suspicious things even though I only did normal, and What is more unpleasant sometimes casinos cannot answer the real reasons from the necessity of KYC to withdraw funds.
I think it is not about only the suspicious activities that the gambling casinos ask for the KYC but whenever you want to withdraw a big amount they will ask for the KYC purposely so that in case you are unable to provide the KYC documents they have the excuse of not withdrawing to your account.
We are living in a era where most of the gambling sites will ask for the KYC so I don't think it is that I suggest to look for the non KYC because most of the them will likely to scam sooner or later. The first thing we should look while deciding the gambling platform is to find out which of the sites are the trusted ones. Once we get to know about the trusted sites then we can choose any one of them which suits our needs and I think we're in the situation where most of the cases KYC will be mandatory.