Games have their own benefits and entertainment value. But gambling with real money and having them at stake based on your wins and losses provide a different kind of thrill.
That is the point. Online games do not involve this element. For those people who love to take risk and are adventurous by nature, these games become boredom after few shots. Risking money, waiting for the results, the hope of winning and the moments before the outcome are what gambling makes attractive to risk takers. In addition to this, everyone has his own taste. So our definitions and means of entertainment vary from individual to individual.