To get really picky though, I thought the official line was that Wikipedia is a *project* to *build* an encyclopedia. Or maybe "Wikipedia" is the name of the encyclopaedia being built by "The Wikipedia Project". In that case, half of the entries at WP:NOT refer to the project (wikipedia is not therapy, a game, myspace, a chat site) and half refer to the actual encyclopaedia (not an indiscriminate collection of information, not a jargon file etc).
To answer your original point though, "why expand WP:NOT"? Basically because the nature of Wiki means that people can endlessly expand the project in new directions. Saying "WP is X and nothing but X" doesn't really pose any limits on this expansion. Saying "WP is X, but definitely not Y or Z" does.
Steve
There's at least 2 sections in WP:NOT. One which explains what the project is not, one which explains what the articles are not. BTW, I do agree with that last paragraph. You can say Wikipedia is a free encyclopedia anyone can edit, but some people don't seem to grasp the idea of what an encyclopedia is, so while it's unfortunate we have to use negative language "Wikipedia is not a vehicle for promotion" is something we need to have in there. Even if it's just so we can point people there. It's easier than explaining what type of content is encyclopedic and what isn't.
Mgm