When teachers do reading they usually divide the students into groups. Each child, and the teacher, has a copy of the book. This would be difficult to do on a computer.

In addition, personalized books may be presented as gifts, put in the classroom library, etc.

Other things to consider as well, screens have glare, a lot of school computers cannot be easily positioned for individual eye sight. Most importantly, in my mind though, is the portability, hands on feature, and easy access.

I hope this helps answer your question.