Nothing forces a casino to be regulated.

They can work under no license at all and accept anyone's bets based on just reputation
 

Not that Curacao or Comoros gambling licenses are worth anything. Their regulatory authority can't get any meaningful enforcement.

Regulation is only necessary if you want to reach a certain market LEGALLY. So because most countries will block non licensed casinos and they see them, then if you want to unlock most of a market it'd be the only way.