Unpredictable or random rewards are more addictive than a predictable schedule.
"item drops on levels are never guaranteed, requiring players to waste stamina on grinding"
"there are spots on the level map that give you a random reward when reached, and completing the daily puzzle gives you a random prize"
"there are a few types of seeds (particularly event seeds) that give you a random pikmin when it finishes growing, so it's impossible to determine when or if you'll be able to finish an event with limited time pikmin"
"So. Many. Rewards. And there's mostly just random and end up being useless. I cannot stress this enough."
"Minigames can be played to unlock random clothing items. These minigames are optional."
"Some events that you need to do repetitive tasks to complete have a fortune cookie as their reward, which gives you a random item (the collection of which depends on the cookie, that much you know from its name and appearance.)"