Why? They mostly don't do anything very impactful. Voting and other civic engagement is very low amongst the illiterate.
I don't believe the literacy rate is a measure of intelligence, when the "three cueing" educational calamity is right there as an obvious explanation. That and Americans just being overworked and exhausted and sick all the time.
we're talking in the area of 100M+ people, remove accessible reading from them and you're left with what? cable news and tiktok?