We have many good teachers and potentially good teachers, but we also have tenure. Tenure guarantees a teacher employment without termination except on grounds of immorality and incompetency. Incompetency in the case of a tenured teacher is almost impossible to prove.
Guaranteeing a teacher, even a potentially good teacher, a job often results in a demoralizing percentage of teachers simply going through the motions of teaching, devising a system requiring the least work and performing at a level that will get the teacher through the school year.
To prevent such teacher behavior, public institutions should have the administrative muscle to evaluate and fire these teachers. With tenure, this is not possible.