It's because it's inconvenient to discuss. What you said is the actual truth.

The really interesting thing was that when looking further, the results of non-native students reflected closely the scores of their native countries (not the countries where they now live), which is not at all surprising to me (as an immigrant, I don't think I am smarter because I now live in a "smarter" country even if I would love to be) but goes completely against everything people in the education sector appears to believe.