Unpredictable or random rewards are more addictive than a predictable schedule.
"There is effectively a loot box system, where you get a random chance of gaining a character. You are guaranteed a base level character, but the higher level characters are much less likely to drop."
"Rewards are given out in variable amounts for completing quests; the amount depends on quest length"
"The time-limited bundles that only appear when you complete a certain task are not fixed at the same value. Some people will get better value than others. This is extremely unfair and disadvantageous to spenders."