Which psychological principle is MOST directly responsible for making loot boxes in video games so addictive and profitable?

Loot boxes have become a controversial but highly profitable monetization strategy in modern gaming. These virtual items offer random rewards in exchange for real or in-game currency. But what psychological principle explains their addictive appeal to players? Test your knowledge about the consumer psychology behind this gaming industry monetization technique!

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Variable ratio reinforcement schedule - unpredictable rewards that keep players opening more boxes in hopes of getting rare items
Mere exposure effect - players become more fond of loot boxes simply through repeated exposure to them in games
IKEA effect - players value loot box items more because they feel they've worked to earn or 'build' their collection
Decoy effect - developers intentionally include poor items to make premium items seem more valuable by comparison