The idea that 21% of US adults are "illiterate" doesn't mean they can't read, by the way:

> English literacy test results from 2014 suggest that 21% of U.S. adults ages 16 to 65 score at or below PIAAC literacy level 1, meaning they have difficulty "[completing] tasks that require comparing and contrasting information, paraphrasing, or making low-level inferences." Included in that 21% is the 4.2% of respondents who were unable to be assessed due to language barriers, cognitive disability, or physical disability.

https://en.wikipedia.org/wiki/Literacy_in_the_United_States

A lot of people aren't aware that the academic definition of "literacy" was changed around 1950 to no longer refer to "alphabetical literacy", which is still what most people think literacy means (in lay usage).

https://en.wikipedia.org/wiki/Literacy

And this particular survey ignored people who didn't speak English but were entirely literate in their native language. Which obviously has nothing to do with the educational system or people's intelligence.