Here's one reason (amongst many I am sure).
I have a relative who was a Professor at Eastern Michigan and got her PhD in Education. For her dissertation she looked at standardized testing in USA as well as internationally. She was in Finland, doing research at a school there when she noticed a discrepancy in their testing statistics, specifically, the number of students being tested as compared to the total population.
Particularly, she noticed that NONE of the children with special needs were taking the tests. When she inquired as to why, the administrator was pretty blunt: "Well, they are not tested because they are handicapped."
I question why the USA is reacting (maybe even over-reacting) to a situation that is not apples for apples.
EDIT: I realize this is an anecdotal story. But the gist is that education in the USA is an inclusive opportunity for everyone and every student undergoes standardized testing. The same can't be said across the world so how can a true comparison be made?