Most likely this is the reason some gamblers out there are so against KYC on all its forms, security breaches are incredibly common, if you take the time to research the topic you will see that even companies like Facebook end up leaking the information of millions of their users all the time, and taking into account how some criminals may use that information many gamblers are fearful their personal information could end up being sold at the black markets, and this could then cause them huge problems on their everyday lives.
I believe any person have a facebook account, it means almost people's identity already leaked in the first place.
Anyway identity breach or bad usage isn't only happen in casino, did you forget you've submitted your identity on a centralized exchange? banks? school and university? companies? someone who claim himself as a police or surveyor? there's a lot places you've share your identities, except you've avoid all of them which is impossible.
It is true that it is impossible to avoid revealing your identity and to have your documents stored on the computer of someone else in this world, but at the same time it is not as if we are going to give away our documents to every single entity that ask us that information, so some kind of balance must be reached, and there are gamblers out there that if they were asked for that information by the casino in which they are playing they will refuse to give them away, even if it means losing whatever money they had at the casino.