The literacy rate actually lines up really well with the high school graduation rate, which is 88%. There are a lot of models that try to explain why American sucks at education, but I doubt it will surprise you to learn the probable answers are poverty, racism, and capitalism.

Underfunded schools, stressed out kids who con't care, parents who are overworked and don't care, children who are shuffled around any relative who will take them. I used to work in education and knew a lot of kids who were about 8 years old living with a bedridden older relative and no one else. It's also a lot of racist political authorities and real estate developers who box all the non-whites into poorer living conditions and deliberately underfund public services. There are some people who also talk about lead poisoning from paint and water in impoverished areas causing learning disabilities and that sounds reasonable enough to me, but I don't know anything about it for sure. Even if it weren't lead poisoning the racism and poverty would do the job the same. A lot of it also might do with America's hatred of immigrants, since something like a third of illiterate children in the US were born outside the country.

I don't know enough about the history of education in other developed, capitalist countries to know why their literacy rates are higher. Might be something particular to American imperialism and expressions of racism, probably a lot to do with America's history of slavery. Although Brazil didn't outlaw slavery until 1888 and they have a higher literacy rate than America too.

Yeah, if anyone else here also has ideas on what causes this I'd like to hear them

just at a guess probably decades of institutionally not prioritising education