Unpredictable or random rewards are more addictive than a predictable schedule.
"The pipes are random rewards. If you come back daily, you also get a Daily Reward, but those are already predetermined and visible."
"There are free loot boxes that give you very little at all, and if you get anything of value it is almost always the least amount listed in the chances."
"When you complete any puzzle, what you get from that puzzle is luck based."