They might be lucky to get away with it in under developed countries with less effective technology but in develop countries with heavy technology like China and the others, catching them is quite easy.
You maybe right, big country has ability to catch those people but in fact, people can always run away and avoid ban. As you mentioned China, let's take Bitcoin as an example. It's banned there, but the citizen still be able trade or doing some activities on crypto space. It means even big country has enough power, they just can't always do anything without thinking twice as they might face a bigger problem later.
They are even so powerful but some of their citizens never find it good, not only in China but all over the world.
Casinos would still exist even they are banned cause they know that many people would like to gamble and living to that edge, casinos owners would find a way in order to run their business illegally. This what it meant trouble in the future. Cause the more they become strick into banning gambling the more illegalities will increase.