That makes sense. Here's three more: (a) They tend to have (or will end up with) great base stats. Since the pre-evolved forms are usually under lv 20, they would have to show up early in the game; even at the rarest the game can get, wild starters would allow the player to build a game-breaker team by getting all three. Also: (b) Since all three starters are different Types, picking one and only one will affect (hopefully) which Pokemon you catch to support and build your team. For example, getting a Grass starter would encourage getting a Flying-type early to defend against the Bug-Types. And: (c) If you could pick all three at once, how can any of them become a favorite? I love my Snivy line, and having to pick just one makes it special.