(goddamn it, I wasn't logged in)
The official rule:
Strike Zone is the area over home plate, the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants, and the lower level is a line at the hollow beneath the knee cap. The Strike Zone shall be determined from the batter’s stance as the batter is prepared to swing at a pitched ball.
What if the robot can't see the top of my pants? (My shirt is loose and blouses over)
What if my shoulders are angled? (Where's the 'top'?)
What if I have loose pants and a locked knee stance? (Where's my knee, and thus the hollow below the cap?)
When does the robot determine the boundaries of the zone? (If it's at the windup, I'll crouch during it then stand up. If it's as the pitch comes in, I'll squat on high strikes)
A living, breathing umpire makes all these subjective decisions on every pitch. There's no way to trick the umpire into giving you a smaller or undefined strike zone.
You have to keep umpires, even if there's instant replay.
For another example: on a large fraction of 6-4-3 and 4-6-3 double plays, the infielder making the play at second base doesn't actually tag the bag. Umpires are very generous on the player touching the base on the turn. Relying on a robot to make that call would be incredibly disruptive with the way that call has been made for over a century.