In some video games, the main character is a blank slate. The Heroic Mime's silence is chalked up to this, and this is often the standard lead character for many kinds of video games, since you, as the player, control the action. The problem with ciphers is being a target for emoting to doesn't make you an interesting character from a story point of view. And if you look blander than everyone else -- which you will be, in order to be as generally appealing and projectible as possible -- you just stand out as boring. See also AFGNCAAP and The Nondescript.