They don't trust the casino as the casino might misuse the kyc documents or the gamblers want to gamble without telling anyone that they are the gamblers, gambling is illegal in their region or religion and they want to hide their identity or there could be any other solid reason?
Most of the time it probably has to do with 1 of 3 things:
1) not wanting to give out your personal information to an unknown.
2) to avoid paying taxes
3) because gambling is illegal in you area
These are probably the main three things that cover 90% of the reasons why.
The first one is the main consideration majority of the casinos are anonymous we don't know the company running the project and of course, as human beings, we do not surrender what we value to people we do not know because it can cost us trouble in the future, this is the internet and there are so many hackers and information sellers and thinking that you have your information floating in the internet or at the hands of people you don't know is a big cause of concern