i do not really understand, why students do not come together then and try to write, for example on wikibooks.org good comprehensive study material. i mean aft

i do not really understand, why students do not come together then and try to write, for example on wikibooks.org good comprehensive study material. i mean after that, and when everybody looks on the project, that would be done? and at least that works for the entry level courses? why does nobody do that?