Because, at the grade school level at least, many (most?) teachers have never taken any math beyond the absolute minimum required to graduate high school.  And while a university degree is required no math of any sort is required for a humanities degree.

That means that grade school math is largely being taught by someone who not only doesn't know much about math but also found math to be difficult when they were in school.

That latter point bears repeating: the teacher believes that "MATH IS HARD".  And if a teacher feels that way that message is communicated (consciously or unconsciously) to the students.

That's a recipe for failure right there - add in all the problems with the teaching methods that we all know about and it becomes a recipe for disaster (not just failure)