It doesn't, but think about it from their perspective. Do you base performance on number of A's given? On scoring on standardized tests that only show how well you taught students to take tests? Where do you include "opening a student's eyes to the power of reason and science" or "planting a seed of a deep love for literature" or even just "being a person a student wants to be like"?
It isn't much different than how've question of how to evaluate coders. Any mechanism that can be easily proscribed can be easily gamed or is meaningless. Any mechanism that truly measures worth is extremely complex, doesn't scale, and may have issues with labor laws.
I wouldn't want that crap legislated by state legislatures, either.
