Maybe because "-foo" means "search for documents that do not contain foo", so queries like "foo -foo" never match anything: https://www.google.com/search?q=foo

Maybe because "-foo" means "search for documents that do not contain foo", so queries like "foo -foo" never match anything: https://www.google.com/search?q=foo+-foo https://www.google.com/search?q=foo+-foo