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... In the long run, of course, the gambler will lose more than win because gambling is a zero-sum game.
Actually, if you consider the casino or the bookie is always supposed to get profit from the gambling volume of gamblers and bettors, then gambling/betting is not strictly a zero-sum game, but rather a negative-sum game, since the house will always have their slice of the pie from the volume of the money gambling in order to continue their operations, pay their employees and keep their facilities tidy and attractive for newcomers to feel tempted to gambler for the first time.
In short, if it was actually a zero-sum game as you claim, the casino would have no house edge and RTP would be 100 percent for all their slot machines, among other things, which we all know are not applied in reality.
Thank you for the correction. Yes, if we play poker among ourselves (without a third party) it is a zero-sum game. If in a gambling game there is a third party (casino or bookmaker) that receives a commission from gamblers the game is a negative sum game. That is why the more often we gamble the more often we lose. This must be understood before gambling. Otherwise, you can get very frustrated because of unreasonable expectations.