Why is it that many academic books are written by a different expert each chapter? I've noticed that many academic expositions are written in such a way that a different expert supplies each chapter, and one editor brings it all together. 

I can understand why such a format makes sense in a "The Handbook of something-or-other", for example, where exhaustive coverage and authoritative material are sought. But I've seen it used, just recently, in The Cambridge History of Western Music Theory, a history book, and in The Psychology of Music, an exposition of an academic field.

My question is: how and when did this practice arise? (I don't recall seeing such a practice in books published more than 50 years ago). What are its upsides and downsides, and what makes it attractive?
Many times such a book is dedicated to a broad subject with many sub-subjects and experts from each sub-subject are invited to write the relevant topic. This is actually very good. The potential downside might be that different authors might have much different writing style and flow and it can lead to a highly non-uniform  book if the editor does not pay the required attention.