I think its less to do with writing per se then general educational apathy/mediocrity/low bars. It's not like students can't do it, it's that they are not required to.

The other thing I'd point out is that the spectrum is just so broad when it comes to educational quality in the United States. There is a big difference between going to Thomas Jefferson or Stuyvesant and the local public high school in some rural town or inner city with a 12% graduation rate...