Things that are _not_ to be found in libraries are published in peer- reviewed journals, not encyclopedias. Wikipedia is not a peer- reviewed journal. And journals have their own requirements, which in fact are more stringent than those for an encyclopedia, one being reproducibility of results. That's the "new-knowledge" equivalent of what we call "verifiability" and it is _much_ harder to do.
I don't follow you... my Uni library contains hundreds (probably thousands) of peer-reviewed journals...
Which is why writing an encyclopedia article takes on the order of weeks while writing a research paper takes on the order of years.
A encyclopaedia article is quicker to write because the only thing you have to do is write it. A research paper requires you to do the research first. Actually writing the paper doesn't take very long.
Things that are _not_ to be found in libraries are also published in popular magazines, books by any publisher that thinks they can make money by selling them, self-published, published on the web, etc. Whether these are to be called "knowledge" depends on which definition of "knowledge" you use. In the Britannica's slogan, "The sum of human knowledge," they likely meant AHD4's meaning number 4, "4. Learning; erudition: teachers of great knowledge." It is, after all, an encyclo-PAEDIA.
Plenty of libraries keep archives of magazines. Any publisher can have books in libraries, whether you like their books or not (which seems to be the only distinction you're making between books that should be in a library and books that should not - all publishers are in it to make money, that's what it means to be a commercial company).