The fact that you can win much money with gambling is enough to attract so many people in this world. People just love money, so the fact that they can win more money with money sounds good to those people.
And the reward you get when you win. It is like when you train a dog and you give them reward if they follow you, then they will keep.doing it. It is the same with gambling, when we win, something triggers in us to do it again. Until it become a habit and then an addiction.