Why? They mostly don't do anything very impactful. Voting and other civic engagement is very low amongst the illiterate.
we're talking in the area of 100M+ people, remove accessible reading from them and you're left with what? cable news and tiktok?
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.