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.
there are many cases of data leakage in my country, it's even true what you said not only in casinos but many more such as online shopping sites and personal information data on government sites, indeed if you look at it there is no safe place on the internet so why worry and oppose this KYC anyway If we look only at one side, for example, a casino that implements KYC and thinks it's not safe, whether other places are also safe, I don't think so, as long as KYC is requested by the casino to prevent money laundering or other prohibited actions, why should we avoid it. if we are the right people it's ok to give that KYC as long as the casino is reputable too. that's in my opinion