Quoting books not included in the Bible

From http://www.infidels.org/library/magazines/tsr/1991/4/4duck91.html:

Furthermore, the "inspired" Jude identified the author of this quotation as "Enoch, the seventh from Adam," so if Jude thought that Enoch had made this statement, he must have been endorsing the Enochian authorship of the book. And if the Holy Spirit (as the doctrine of verbal inspiration teaches) was directing Jude in what he wrote, then the Holy Spirit must have actually known that Enoch had written the book. The inerrantists, then, have nowhere to go except to conclude that 1 Enoch was actually written by Enoch, the seventh-generation descendant of Adam who "walked with God" and "was not, for God took him" (Gen. 5:24). So if bibliolaters are looking for something to give credibility to 1 Enoch, they surely have it in these facts. They have to believe that the book was written by a man who was so righteous that he was translated directly to heaven without seeing death (Heb. 11:5). Yet with all that to commend it, the book is not even in the holy canon. But that's another article for another time.

Why would a book in the Bible quote directly from a book not included in the Bible? If it's inspired enough to quote directly, then why wouldn't it be included in the canon?