According to the Pew Research Center, the number of kids who are reading just for fun is at the lowest it's been since 1984. Kids aren't reading for the sheer enjoyment of reading anymore.
I would argue that it's because we've trained them to hate books. In our proclivity to drill and kill with worksheets in order that we might have concrete written proof that they've not only read the material but that they fully understand it, we've turned pleasure reading into an assignment. It's no longer fun anymore.