Because children who are below grade level by age ten tend to stagnate and eventually give up and drop out in high school. Harvard educational psychologist Jeanne Chall famously called this phenomenon the "fourth grade reading slump," where children cannot make the transition from learning to read to "reading to learn," which hinders their learning in all other subjects. Because of these early literacy setbacks America is losing the global race in science and math, areas central for 21st century skilled jobs. (Michael Levine at Huffington Post)
The Michael Levine piece referenced above is a call for digital literacy to be taught in school, in effect, a plea that we really do acknowledge the digital age in which we live.