This isn't the worst theory imo, the biggest misconception is gambling isn't and will never be a source of entertainment. I did understand your theory and it might make sense but the problem with most people is they spend a lot on this so called entertainment. A movie ticket might cost maximum of 30$ but for many gamblers they want to turn 30$ to 3000$ which is why they get addicted easily
I can't completely disagree to what you said. However, a lot of people take gambling as a form of entertainment. Entertainment is subjective in nature. Hiring a hooker is also an entertainment to many. People spend money on that to get something in return that relaxes themselves. Similarly gambling relaxes a lot of people and they do it to get something in return. Probably you prefer to spend on movies to relax yourself. Some other people prefers gambling. Addiction here is completely self inflicted.