Let me start by saying there are some very good teachers but, keep in mind that most of the people who chose Education as their college major graduated in the lowest 25% of their high school class. After a few iterations, you have the dumbest people who went to college becoming the teachers of our current generation of students.
And, even idiots go to grad school and graduate with advanced degrees. They come out believing their idiotic graduate theses are valid and they get employed as school system administrators where they enact their "advanced" ideas. And we go downhill from there.