It does exist, it is just incredibly painful to use -- it's a raw SQL dump of each individual page, which is missing some of the dynamically generated parts of the page you might care about (like categories, unless you separately download that table too). You also have to then run it through wiki software locally if you want the rendered page instead of markup.

Don't get me wrong, it is wonderful that the Wikipedia team offers this, and I am grateful they give anything at all for offline usage. It just feels like it's intended more as a side product of their backup process, rather than something you're really supposed to use.