Because they would become monstrous behemoths that did everything at a lower quality. For instance - Mixcloud and Soundcloud each serve a real purpose, separate from one another. It may seem like they do the same thing (connecting people over music) but really they have refined their respective systems around what they are built for - focus on single tracks and the other on music sets. Now if google started doing that, it would not be anywhere near as refined because their platform would have to serve other purposes; if facebook started hosting repositories it would be diluted with everything else they do.