Probably because most people are satisfied with the functions offered by Google. There are documents that lie in the clouds or Google Docs. Periodically, someone tries to do a search there.
But this shop quickly closes and is regularly searched only among the documents that are posted on the sites for all in the public domain.
And the search will be conducted only in the text of the documents of the .PDF format. In the query string, enter filetype: doc.
And the search will be conducted only in the text of documents of the .DOC format. And so on. Enter the required format, for example, .JPG.