Since "No Child Left Behind" came into effect, it's nearly impossible to hold kids back, so many just keep advancing through the grades even though they can't read or do basic match. There's over 2 decades of kids that were pushed through no matter what to make the numbers look good. Obviously not true of all kids, parents and exceptional teachers can still intervene when the system fails a student, but a large amount of kids coming out of high school fell behind a long time ago.