Since so many of you are trying to explain how we got to where we are today with the rule, many of you are not going back far enough into the history of the game.
We are talking 1870's era ball, this was when the batsman was allowed to declare, "high", "low", "neither" as his preferred striking zone. It took 9 balls for a walk and 8 strikes for a batter to become "struck" (BTW the umpire had the option at the time to declare a pitch neither a strike or a ball).
This term "struck" is where we get the "K" from for a strikeout, because the "S" had already been used for the sacrifice.
I could go on for a page or so to explain why the rule is what it is today but to really see why here is a link going over the rule changes since the game of the 1840's
Baseball History: 19th Century Baseball: The Rules
Jasper