That's true but these days gamblers don't care about anything and even if the site leaks their data they don't care about it. They are intentionally doing KYC on those sites because most people think that KYC is a necessity on the sites where you can earn money. The same case is with the users who have their accounts on crypto-sites, they are also doing their KYC because it has been feeded in their mind that KYC is necessary to get your money and earnings into your bank accounts. Even those sites that have been in industry for more than 2 years or more can sometimes misuse the KYC data. I have heard that KYC data is getting sold for solid rates on black markets, and if that's true then doing KYC on any website is unsafe, and if a user does that then they are willing accepting the fact that their data can be leaked or hacked and will be shared with many people.
You said it right, after knowing that there's a chance that your data can breach and might be sold out into black market and you are still willing to complete and submit it, only proves that you are confirming and you are accepting what the casino is requiring you, whatever happened in that information you can't blame anyone.
As it's a part of the user and agreement where you accept that there's nothing you can do whatever happened to your data, you submit it with that will have accepting and acknowledging that said rule.