Wikibooks doesn't reward the authors at all. Wikibooks doesn't pay them money, it doesn't put their name on their work, and it doesn't let their contribution last forever -- others can go and muck it up with their editing. (If anything I've said here is wrong, then let's say it's the perception of these problems that is the problem.)
Also, the current books aren't particularly good and the syntax for editing them is arcane.
Project Gutenberg, Flickr, and Wikipedia (to cite just three examples... there are many more) don't pay their contributers money. On Wikipedia contributions can be mucked with by others. Yet these sites are all wildly successful.
Short articles on a specific topic require little coordination among contributors. A book used for class can't have redundant sections. Each part needs to build upon the previous sections (for math, anyway). The book should have a consistent style.
Also, the current books aren't particularly good and the syntax for editing them is arcane.