Game Manuals are great. I actually kind of enjoy reading them to learn little facts about a game. (I'm weird, I know.) However, with a game as large as WoW, it wouldn't need a manual so much as an encyclopedia series! And then, you have the issue where it will quickly become out-of-date. Heck, I remember reading my Vanilla WoW manual and seeing something about "you will gain dishonor for killing guards", and by that time that feature was long gone.

Even if they made an updated one at the start of each expansion, it wouldn't be enough. There'd be no mention of hardmode raids in Wrath, it wouldn't mention the option to change fur color for Tauren at the barber shop, it would still say Unholy DKs have a talent to revive themselves as a ghoul upon death, Frost wouldn't be the "dual-wield tree", no mention that hunters no longer gain any special bonus from quivers, etc.

In a game that changes as rapidly as WoW, the best way to distribute up-to-date and accurate information is via websites, and even if Blizzard DID do that themselves, there would be plenty of fan-created websites doing the same thing, anyway.

Besides, with a game as large as WoW, even veteran players can learn a thing or two every once in a while. That's part of the fun. =)