Because, the fundamental issue with education isn't the quality of teachers. It's the home lives of the students. That's why there's such a big disparity in public education in the States. Until we fix the cultural and social issues, schools will continue to be at the level they are. If we want to fix education, we need to fix the underlying problems that cause some kids to not do well. And it's generally not the teachers (yes, there are bad teachers; but socioeconomic factors of the kids have much more impact)