After Las Vegas, Macau is the most visited place for gambling. If not mistaken even gambling constitute to 85% contribution in their economy. So it is all gambling and the tourist that comes to visit Macau. Also, people who cannot afford to travel Vegas, ensure that they do visit Macau to enjoy the gambling and nightlife out there. Since China and Macau are quite close by so people do visit on Friday night and then go back to China on Sunday.
Obviously Macau is one of the global hubs for gambling. But the problem with Macau is that the casinos can be very expensive and not everyone from mainland China can afford to go to these casinos and indulge in gambling. That's the reason why a lot of them look for more affordable destinations, such as Vietname, Myanmar and Cambodia.