Research moves fast though; a dissertation just 3 or 4 years old may already be hopelessly out of date. Also, they are written by PhD students who, while masters in their own field of expertise, are really only 'apprentices' in training and many not be very knowledgable about areas only slightly outside their domain.

Scientific journals often publish 'review' articles where people with decades of intimate knowledge about a field summarize recent developments. They are usually more concise than dissertations, and often written much better too. They are also peer-reviewed, just like dissertations and other papers.