I don't know if "theoretically" is good enough here. The theory hasn't really been tested or proven. It's more of a hypothesis, and an ideologically driven one at that.
My theory is mostly that "gambling" seems like it inherently means "buying stocks based on something other than their concrete value". More gambling means stocks drift further from their "true value", which means a higher payout for correcting them back to what their price should actually be.
The whole thing does get very fuzzy because of the "market can stay irrational longer than you can stay solvent" aspect. It's not enough to know what the correct price is, you have to know when other people will realize that as well, or else convince people that your price is "correct".