Good point - random or unpredictable rewards are known to be compelling/addictive for rats, and the same trick seems to work on most humans as well.

Though as I understand it much of money in gambling is made from "whales" - players who lose lots of money and keep playing anyway. The same term is used for f2p game players who spend a lot of money on in-app purchases, often tokens for virtual slot machines for desirable in-game items.