I was always under the impression that publishing a book was more prestigious than publishing articles so why would someone write something with the quantity to be a book and prefer to publish it as a journal article? Are there hidden benefits of publishing as an article over publishing as a book?
As always, it depends. First of all,
depends on the area. In parts of the humanities, writing a book is an almost-necessary condition for tenure, but in my area of CS, a book is viewed as best as a waste of time pre-tenure.
In terms of absolute prestige, books and articles serve different purposes. Even a 200 page article might be a technical exposition of a single result. Witness for example the ongoing series of articles on graph minors in mathematics. A book often tries to distill and put in perspective a body of work, and (sometimes) might have educational components like exercises/problems.
A well-written book gives you some cachet as an expert in an area, but a seminal journal article can do the same. It really depends on what purpose you expect the document to serve.
A final point. A book (even a technical one) may not be peer-reviewed in the same way as an article. Indeed, it's not common in my area to have brand new research appear first in a book (I'm excluding simple observations and recasting of results).
This depends on a number of factors (the field, the scope of the work, and the reputation of the publisher), but a book often summarizes the current state of a field, while an article adds something new to the state of the field.
So, one of these (the book) estabishes you as a subject matter expert, while the other (the article) establishes you as a subject matter pioneer. In institutions where research is highly valued, it wouldn't surprise me at all to find more prestige associated with article publication.
That said, there are exceptions, of course: A textbook that becomes recognized as a standard in the discipline might garner more prestige than an article in some lightweight publication. Unless the book happens to be based mostly on your own research, though, it's easy to see why a textbook might not gain you too much reputation in an environment striving to be known for their cutting-edge research. One is more forward-looking while the other is more backward-reaching.