Let's face it: in Real Life, if a weapon is powerful, it's mass-produced. If an animal is powerful, it flourishes. If a nation is powerful, it can effectively Take Over the World. That's not the case with games, though. Generally, for balancing purposes, the more powerful an item/mon/etc. is in a game, the harder it is to find. While various reasons are given in the plot (if at all) as to why these things are so rare, in the meta-sense, it's for balance. If the player has unlimited access to game-changing stuff, then the game is tipped entirely in their favor.