Because the truth, as I have discovered it, wouldn't sell books. It would be boring and depressing.
The truth of baby sleep is this: their little brains are developing, their little bodies are developing, and their sleep habits are constantly changing.
It doesn't matter what you do, if you let the poor little dear cry until they vomit, come console them at regular scheduled intervals, snuggle with them in a family bed, or some hybrid of the above. They're going to do what they're going to do and not consult the adults whose very sanity lies in their precious little hands as to what would make those adults happiest. Sometimes they're going to go down easily at 8pm and sleep quietly until 7am, and sometimes they refuse to even consider sleep until 10pm, and then wake up at 1am and 6am.
The one thing I have found is that snuggling seems to keep the crying at a minimum, and I just don't see how letting the Bug cry it out would change anything, except possibly to get me put on blood pressure meds. I have friends who are firm believers in cry-it-out, and they deal with the same sleep issues we do.
The problem with the sleep books is that they lead a mother to believe that she is a moral failure if her child doesn't sleep a beautific 10 hours at night and take two easy two hour naps during the day, promptly at 10am and 2pm.
I am not a moral failure. Those guys (and they are, for the most part, men) are just assholes.