I'm not quite sure what you're asking about so I'll expand on this thought. The reason that the batter's knees and upper body are used to define the bottom and top of the strike zone is because there are no other physical reference points near home plate that an umpire can use to determine the distance above home plate of a pitch as it passes over home plate. It would be simple to define the bottom of the strike zone as being, say, 16 inches above the plate and the top of the strike zone as being, say, 42 inches above the plate but there's no ruler there for the umpire to use. This means that a short batter has a smaller strike zone than a tall batter, with additional variability introduced based on a batter's stance. The systems for calling strikes rely on cameras at multiple angles and can be programmed with any upper and lower limits for strikes. Having those limits based on the individual batter means that each batter would have to have his strike zone programmed into the computer and the system would have to be reset each time a new batter bats. While I assume this would not be difficult it introduces a risk of error. Having a uniformly-defined strike zone for all players eliminates this risk and, in my opinion, is much more fair than the current definition.