I guess it's just a matter of infrastructure cost (big) overshadowing profits (small).
That being said, Scribd does a GENIUS SEO trick that I haven't seen anywhere else. Scribd tracks all search terms that lead to a document, and then embed a link in that document's page with the search term as the anchor text. To see this in action, go to a Scribd document (http://www.scribd.com/doc/918737/Ruby-on-Rails-Simple-Exampl...), go to the "Indexing" box in the lower right, and scroll through the list.
That being said, Scribd does a GENIUS SEO trick that I haven't seen anywhere else. Scribd tracks all search terms that lead to a document, and then embed a link in that document's page with the search term as the anchor text. To see this in action, go to a Scribd document (http://www.scribd.com/doc/918737/Ruby-on-Rails-Simple-Exampl...), go to the "Indexing" box in the lower right, and scroll through the list.