My cousin is illiterate. Why has he gotten so far in school?

I have a cousin who is almost 10 and is still functionally illiterate. His teachers and his parents are aware of this, yet he has still managed to be promoted to the 2nd or 3rd grade. WHY?!!!! He obviously isn't ready for that level of schoolwork, so why has he been allowed to get this far? He has been diagnosed with ADHD, and when he takes meds, they seem to help him focus and learn better. Unfortunately, he still hasn't caught up far enough. And don't get me wrong. I'm not just blaming his school for this. I also blame his mother, because she practically neglects him and lavishes all of her attention on his elder brother. I know that my cousin isn't the only functionally illiterate person who has been promoted from one grade to another when he shouldn't have been. Why is this happening, though? Why are so many children being shortchanged by being inadequately educated, being promoted from one grade to the next, and being allowed to graduate high school?