Because the crucial thing about lootboxes is that in-game currency obfuscates the actual value of your purchase. You know when you buy a booster pack how much that booster pack is worth in real life money; you don't when you're buying lootboxes. When I was a kid and bought booster packs I knew how much they were worth because I also knew the value of each card and how much they compared to other physical stuff I could buy (candy).