Casinos know that if they asked for this information upfront a great deal of players will not bother... I think casinos keep this information hidden so scammers do not abuse their policies
I agree with the first part and I totally understand why most casinos do not enforce kyc on registration as this will result in them losing many of potential customers. But the reason you mentioned isn't reasonable and some may even consider it unethical since a good service provider has to as clear and transparent as possible with its customers when it comes to their policies.
the strange thing in my opinion is, many crypto gambling sites currently do not enforce KYC at the time of registration but when WD they do it, it is clear that users are like being deceived by their tricks, I have never used gambling sites that require KYC, there are many sites gambling that does not apply KYC and respects user privacy.