Simply put it is because in the US the quality of teachers is fairly low.

For those of you who haven't down voted already I can explain why this happens:

Its a low paying job, the only reason to go into teaching is passion. Its great that folks are motivated, but that doesn't make them GOOD. Plenty of people pursue careers in art and music and fail, few people who pursue teaching get pushed out by "better" teachers.

Give someone a tool that lets them take the easy way out and not have to exercise critical thinking skills and this is the outcome you get.