Why do academics write/publish book reviews for journals?
In my experience, reviews are written at the request of journal editors and are typically not initiated by the person writing them.  My colleagues and I usually write several book reviews every year because our supervisor is on the editorial board of the journal and tells us to write them.  As reviewers, we do receive a free copy of the book, if you consider that an incentive. There is no prestige associated with writing one, and they don't count as publications in our annual performance reviews.