Because there are things on a baseball diamond that can never be taught in a classroom. Oh, some have tried. Phantom tag plays, neighborhood plays, strike zones of least resistance - all of these are techniques to teach umpires how to call the game that everyone else sees. Because it is the game that everyone else sees that is the only true reality. It would be futile to convince the whole world that they're wrong.
Reality is relative.
Reality is relative. The same is true on a baseball diamond. If everyone sees a pitch as a strike, well by golly it's a strike. This isn't totally about game control. It's about doing the best job we can as umpires.