This situation is almost certainly true for popular nonfiction authors as well, but as I don't follow blogs or other PR mechanisms for more than one of them, I can't say.
What it doesn't seem to be true for is academic authors. Academic books generally have small runs and appeal to niche audiences. Many academic libraries will automatically buy all volumes from the right press or series. Many academic books are not priced to sell - they're priced to keep a press alive and maintain small print runs. Presses don't provide much PR for academic books either, but then neither do the authors.
There are a number of reasons for this. Academic authors know what the market's like and are less likely to publish in the hopes of income. I suspect there are fewer multi-book contracts in academia. Each book is likely to be negotiated separately, especially since few academic authors are able to produce new volumes very frequently. One book every few years is a high rate of production, given the necessary research involved.
Additionally, it matters less if books sell slowly in academia. Book reviews regularly appear in academic journals several years after books are published. Fiction needs a prompt sales response, one measured in weeks or months at most. How a novel sells in its first few weeks is critical.
There's the additional complication of job requirements. Academic authors with academic jobs usually have publishing requirements which are prerequisites for promotion. Publish a book, get a job. Publish two more books, get tenure. There is no formula for success so straightforward as this, but it's not that far off either. Promotion is the reward for publishing, which is why academic publishers can get away with token advances, if any.
Because publishing is a prerequisite of promotion, I'm under the impression that many academics err on the side of modesty when it comes to book promotion in order not to intimidate friends and peers. Indeed, one friend of mine regularly alienates her peers by being entirely honest and straightforward about her accomplishments.
One lack of incentive for academic authors to promote their work (whether articles, brochures, or books) is the problematic balance between kinds of books. For promotion, many universities will only count certain kinds of books. Popular nonfiction and textbooks (i.e. the more likely money-earners) often are not the sorts of books which count. Books aimed at a niche academic audience do count, but there is less of a market for them in the first place.
Should academics promote their work more? The market, social tensions, and slow review processes won't always cooperate. But what's the point of writing a book worth publishing if the author doesn't believe in it enough to tell the world about it?