Consider that as of 1800 or so, perhaps 5-10% of the population were literate. That climbed to 90%+ throughout much of Western Europe and the US by 1900, but the level of education was still low: in the US, a high school diploma was an accomplishment only 6% of the population realised in 1900. That climbed to about 90% by 1950 or so. By contrast, more people have graduate degrees today.

Though the content and quality expressed by education ... has shifted. On the one hand, there's clearly been advances in knowledge and education, but at the same time, those are being presented to a much, much larger share of the population.

I've seen people (children, students, professionals) with widely varying levels of literacy and cognitive skills, ranging from frighteningly high to almost none at all. I think this may be underappreciated.

Or, TL;DR: yes, a lot of people are terrible at reading.

it wasn't required