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.
While I think in a similar way, private businesses keep a great deal of their internal procedures a secret from the public and it is their right to do so, a casino may decide to be completely open about the reasons behind their KYC requests but another one may decide it is disadvantageous for them to be that clear as scammers are experts at finding flaws in any system and take advantage of it, so they prefer to keep some of those guidelines a secret to catch scammers and accounts with suspicious behaviors with their guard low, and at the end we are the ones that need to decide with which casino we feel more comfortable and we will use to gamble.