Because we are living in a world essentially barricated by the walls in our minds. And these walls are things we lovingly build brick by brick!
When we read, we walk into the mind of someone who may have died 20,200,400 years ago and know how he thought or what he felt. And this has the potential to change our lives.
Reading is our sustenance, thinking is our exercise. But our memory,however, is our namesis as it plays tricks on us all the time.
If what we read is not retained in our mind, we might not have bothered even to read it!!