Developers use "weighted selection" to control drops. For example, a "Common" item might have a 1/50 chance, while a "Legendary" is tuned to 1/5000.
This occurs when a card that was previously easy to get is reprinted at a higher rarity (e.g., from Common to Secret Rare) to drive sales for new sets. Rarity
Secret Rares and Full Arts (often limited to roughly 2 per case in Pokémon ) serve as "chase cards" that maintain the value of a set for collectors. Developers use "weighted selection" to control drops
In TCGs like Magic: The Gathering , Pokémon , or Yu-Gi-Oh! , rarity is a business tactic as much as a design choice. Rarity