Either the government legalizes gambling or not people will always abuse a setup system, forbthe fact that we have a large number of gamblers with everyone competing to make winning from a casino and some begin to device means to cheat abd byepass the process to the normal protocol, we have to consider that gambling is also decentralized in the sense that no single authority in charge of every gambling activities, so anyone can decided on coming in with something new to offer gamblers irrespective of the circumstances attached.
If gambling is already legalized, there is no need for an illegal gambling anymore however there are gambling operators who can still be considered as illegal if they don't get a permit or license to operate. They do this because maybe processing one will take time and can cost them more money.
Also, these people already have bad record and they will get busted for this if the government will do a background check about it. Gambling itself is not decentralized but if it was a crypto based gambling site then it is possible but gamblers must be careful with this as some can label their casino a decentralized or web3 and still ask for a KYC later on. I already saw some casinos like this.
some operators go to the route of not licensing their site for reasons like, they don't want to pay the fees or they don't want to disclose their documents to the government or they don't want to go thru the process. whatever it is, if it is physical casino, hard to operate without a license as the authorities will always come knocking at your door. easier when it comes to online casinos because they can easily disappear without a trace. this is the dilemma of most gamblers when it comes to online casinos. if it is new and no reputation yet, you can't trust your funds as they can confiscate your fund without reason. you will always find a fly-by-night online casino. but in terms of physical casinos, hard to find unregistered one as it is hard to operate continuously without being seen by authorities.