The other reason why some gamblers do not want to pass KYC is because people worry that it will bring bad effect especially those who live in a country where gambling is illegal.
It may affect their real jobs if their documents are revealed online or maybe something even worst may come (something related to official law of the government).
Coming up to reputable casino wont sell our documents in the dark web, we also need to remember that reputable casinos may have many internal staffs and some of these staffs may become naughty staff by selling the documents for money.
Money can change good person to bad person especially when it comes to big amount of money so documents sales may happen in any casinos if there are some naughty staffs.