Unless you make less money writing the book than you would consulting.

Yes, the content is already produced. But putting it all together in a book format still takes time. You need to go over old articles, choose the ones to go in, edit and rewrite where necessary, figure out what order to put things in, add new content, deal with the publisher, figure out typesetting things, etc.

If that process takes him two weeks and earns him $5k but his consulting rate is $5k per week then writing the book costs him $5k in potential income. In that case, writing the book is actually leaving money on the table.