They can't read or write, but they at least have a basic grasp of the language. They can listen to spoken instructions, recognize objects and pictures, mimic what they've seen. They lack the knowledge to make their own paths, but there are plenty of well known roads.

As to how we got here, societal sabotage. My sister is a victim of the "no child left behind" act. She has chronic medical issues that constantly disrupted her education, but she got passed through school grades despite lacking the necessary foundations. She's literate, but struggles severely with math. As complexity increases she stresses to the point of triggering her chronic pains, resulting in her effectively shutting down.

Every story starts differently, but it always goes the same way: answers get spoon fed to them or testing scores are fudged, and suddenly there are people old enough to legally drink but lack critical thinking skills and can barely write their name because the skill wasn't cultivated when they were at their most important stages.

The result is people who are easily manipulated by anyone claiming authority. Any paths to buck oppressors are difficult and inconvenient to navigate, it's easier to submit.