Because teaching is not necessarily/directly related to learning!
Someone can have all the knowledge, and tests to prove it objectively - yet may not be a good educator. Whether there was a successful transfer of that knowledge, whether learning actually took place, is best assessed on the “other end” of the equation!
If all teachers of one school have similar educational outcome, then you look at the school, the school system, and similar factors - if the outcome varies schools in the same district, that can tell you something, if it varies between teachers of the same school, that again can inform you.