Not allowing humans to call balls & strikes would significantly change the game. There are tons of "unwritten rules" about giving the pitcher an inside strike or a low strike (if the pitcher can consistently hit the same spot, the catcher will be lobbying the ump - many times successfully, but if he's just getting lucky every now and then the catcher will keep quiet about it and hope for the best. Etc, etc. etc). I don't hear anyone inside baseball calling for this change. Plus the rules on the upper and lower limits of the zone pertain to the batter's body parts, which can be interpreted differently by different umps, and how the batter wears his uniform. Who gets to program the computer on where the bottom of a particular batter's kneecap actually is? MLB gives guidelines to the umps, similar to the NFL's ever-changing referee "points of emphasis".