Chess is a great game but it is rare to play for money because you have a very accurate rating system and no luck, so you know to a high degree of certainty who is going to win before the game starts unless the players are very close in rank. You can’t fool yourself easily.

With games of skill that also have a strong luck element (poker, fantasy sports, betting horses, etc.) you can fool yourself very easily into thinking the odds are in your favor when they are not. If you won you thought it was because you played better, if you lost you got unlucky, and if you don’t track it (even many professional players don’t) you may have a hard time even knowing after a long sample size where you stood.

Gambling relies on the ability of anyone to get lucky and win on any given day mechanisms rely on inconsistent rewards.