First, because the Bible is way more difficult of a book than sermons make it seem. We want our Bibles to be like children’s Bibles, full of stories and to the point. However, it turns out that this millennia old volume was written to ancient people, not us. It is full of genealogies, laws, architectural notes and rantings about ancient politics. It has an extraordinary amount of cultural detail that isn’t explained. Most people who pick it up to read it cover to cover give up by Exodus, if they even make it that far. Frankly, its much easier to pay someone else to study it and explain it to us, like churches have done for thousands of years.
Second, the Bible isn’t necessary to live a spiritual life. Love is, Jesus is, God’s spirit is, but the Bible isn’t the only door to open up a person’s spirituality. For some people it really works and for others it causes them to kill their spiritual life by diving into arguments about detailed interpretations.
No one should complain to people about not reading their Bible. They should complain if they fail to love as Jesus loved.