We don't live in a zero-sum economy. Creating a business worth X amount of dollars does not mean that my competitors automatically lose X amount of dollars. Just like the plow increased everyone's harvest, innovations can increase the wealth, time and value of everyone involved.
But, gambling's different. Let's say I enact my gambling theory from yesterday. I get great at poker. Play people worse than me. And win lots of money over time.
All I'm really doing in this situation is siphoning money off idiots. I'm not earning money by adding value to anyone's life. I'm solely taking it away. If you can get past that, it's a no-brainer. But, it's a tough one to get past.