I think. The data they hold and risk losing is more important to gamblers than the money the money they have,
Is it? Do you think casinos will retain customers if they have lost their hot wallet or even the entire bankroll but kept all the user data, including KYC documents safe?
For both parties, the secuirty of funds is important which doesn't mean they don't give importance to the documents.
Some casinos even share the user data like email for promotions with third parties.