Because they hide most content behind a paywall (even content they don't own) and still allow it to be indexed by search engines. It is really frustrating to s

Because they hide most content behind a paywall (even content they don't own) and still allow it to be indexed by search engines. It is really frustrating to search for a more elusive document and find it on scribd only to discover its behind a paywall. They break down search engines by allowing indexation of unavailable content.