sales figures are confidential
Really, is it okay if I ask why?
The only answer to that is, "Because it's always been done that way."
As you note, in many other entertainment industries, and some non-entertainment industries, sales numbers are readily thrown around. A lay person can easily get sales figures for comic books, magazines, DVDs. You can get a fairly good ballpark number of the number of people who went to see the latest theatrical blockbuster. Television ratings are widely known.
What sets these apart from books, in my view, is that there's a certain amount of competition. Companies want to shout their triumph over their competitors. And in comics and movies, the audience culture is such that hard numbers
encourage fanboyism -- "My favorite comic sold better than yours," "My movie stomped your movie into the dirt!" -- with fanboyish trash-talk.
The book industry isn't like that. Ian points out that books, as a whole, are far more diffuse -- and more mainstream -- than, say, super-hero comics. The pond is bigger, basically. And book readers are a little more genteel. Not a lot of trash talking goes on in bookstores.
And it's not that hard numbers for books don't exist -- BookScan compiles book sales figures. But the BookScan numbers like
Fight Club,
no one talks about them. (Also, BookScan numbers reveal that what everyone
reports as best-sellers actually
aren't; classics like Jane Austen and F. Scott Fitzgerald sell, in aggregate over the year, more than the "official" best-sellers do.)
The code of silence in publishing is an ingrained, cultural thing. The New York publishing world would have to collapse and the ground salted so nothing would ever grow there again before it would change.