Well maybe is the answer. Maybe.

First of all, all PhDs can generate some refereed journal articles. But not all PhDs have enough in them to become a book. This is because the PhD and a book have to do different things.
The test of PhD is that it makes a contribution to knowledge and is judged to do so by examiners. Their judgments are based on the way that the research question is developed, the research is designed and conducted, and on the provision of an explicit audit trail of every stage of the research process. The research must be presented as trustworthy, grounded in a thorough survey of literatures and argued as a contribution to knowledge.
An academic book however needs to do both more and less than that – it has to be grounded in good scholarship but it must also have depth and breadth and say something significant about a subject that is going to interest enough people to constitute a market.
Very often a PhD doesn’t have enough to say – yet – about the topic, and more needs to be done in order to make it a book. Take for instance a PhD which develops and trials a model of professional development – it has been tested out on a few people, and this is enough for the PhD. It isn’t enough for the definitive book. In order for this to be a book, the author needs to work with the model more extensively in a range of settings so that it can be demonstrated as viable. The post doc is the place to do this or a carefully staged research agenda immediately after the doctorate is done.
Sometimes the thesis is just too specialised to be a whole book, but it might be a chapter in an edited book. Take for instance a study of reading by a group of children with hearing problems. This is certainly likely to make a PhD but only a chapter in a book on reading, or a chapter in a book on children with hearing problems. Eventually another project will yield a book, and the PhD will be a stepping stone to it.
Sometimes the author just needs more experience. It’s hard to write the definitive book on narrative research for example if you’ve only done one or two projects and you haven’t been doing research for very long. Those definitive best-selling research methods books are usually written by people who’ve been in the game for a long time. People trust them and buy the books because they are experienced. However you might consider putting a proposal for an edited book forward on the back of some innovative methods work because this allows you to get your name put in the same space as those who have the street cred.
But sometimes PhDs do make really good books. Some of these are designed with a book in mind ( I did this and made sure I had a big enough sample of schools to work from to make the argument credible); others just end up having a book in them. Books from the thesis are highly likely to combine a sophisticated theorisation with a serious empirical study that works as an exemplar – see for example, books of PhDs by Richard Niesche, Mary Lou Rassmussen and Valerie Harwood.
Occasionally , there is of course one that bucks all of these patterns!!

The second answer to the question is that, even if the PhD has a book in it, it isn’t actually the same text. In fact you can’t really cut and paste the PhD into a book at all. And publishers will be the first to tell you they don’t want PhDs. This is because PhDs have to do the question posing, methods developing and audit trail work that a book doesn’t.
The book from the thesis is in reality a whole new text which starts with the set of findings to communicate, rather than with a problem to solve/question to answer. The argument in the book is thus constructed differently than in the thesis. As well, the tone and voice is generally different (less formal) and most of the audit trail is removed. (Yes removed, even in scholarly monographs!!)