Some come from countries where literacy is not revered.
Some attended schools that were more like prisons than they were like schools. Some came from areas of the US and times in our history when it was easy to avoid going to school altogether, and there weren't competent adults around to make them do so.
Volunteer at your local literacy project if you want to know more. From the project itself, you would learn a lot more about what to do about it, than how it happened. But when you tutor someone, you learn their story.