There are basically two reasons to avoid KYC:
1.) Taxes. Some gamblers do not want to pay them in principle, and some run away from the laws of their countries with insane tax rates. I understand both of these positions.
2.) Bypassing the bans of states that think they know better than you what you can and cannot. In many places gambling is prohibited, so people from such jurisdictions are looking for ways to satisfy their desires.
If a gambler is avoiding kyc because he is trying to avoid taxes, that is very wrong, it is done in a way if someone is losing and gain but not up to what he lost, no tax will be paid at all. But well if someone lose and not gain, the government has nothing to pay in return, this is another reason how the world is not balanced.
These are the two reasons why people do not like kyc, but it is not only the two reasons, some people just do not like kyc because of privacy reasons which is really worth it.