Here's an article with more details about the study: https://www.apmresearchlab.org/10x-adult-literacy#:~:text=by EMILY SCHMIDT | March 16%2C 2022&text=This means more than half,of a sixth-grade level.

Dr. Iris Feinberg, associate director of the Adult Literacy Research Center at Georgia State University, points to under-served communities with "print deserts," poorly funded schools, and little internet access as being the places where the people with poor reading skills live. She also called it an inter-generational cycle of low literacy, so it's not just a recent problem with people not wanting to read.