The quick answer is because literacy is a culturally dependent set of practices--not a wall.
You can read more about English tests, love letters, and shifting borders in the current political context in my guest blog at youth circulations.
The Americanese wall, 1916
Youth circulations is a "nexus for research, art and activism around youth mobility and the politics of representation" curated by Professors Michele Statz and Lauren Heidbrink. Check it out.