Unlike other games, Casino is a game of luck. And people still believe in getting rich through luck. So it is quite natural that there are some of the websites and the places where this game is being offered. Also the popularity of this game has created more options for the companies who operate such games.
Casino games are fake. Nobody makes money from those games other than the company itself. The reason being the algorithm of such game is designed in such way that you never win. They create false winners and fake proof to populate their website with users. In reality these games keep tight control over how much money you spend. Also some of the times such companies have found to misuse the credit card information.
So these companies are often shady. That is why US has some laws about such sites operating within some of the states. Now that some casino sites are hosting themselves from the middle east and other places where you can't legally sue them, there is more chance of game being rigged.
And for this reason they do more promotion to bring new guillable people into the system. So there are lot of ads on adsense, youtube and other places for the same. These ads can be highly paid or low paid depending on their campaign.