This was probably done because Duel Monsters was released before the first OCG set, Vol.1 , though it is not known why so few of the cards in the game have passwords, especially since the game credited at least nine individuals, but only six of them had passwords, and since the game includes 15 secret monsters, but only nine of them can be unlocked with a password
And thats only the tip of the iceberg
As a 1/1 for a single green mana, it's not likely to affect the board based on its stat value relative to its cost
Theres been talk about Wizards doing more of this recently, but for now, its incredibly rare
Size of Effect Rare gets to have big effects, but we tend to save the largest effects for mythic rare
Gains 300 ATK and DEF for each "Harpie Lady" monster on the field