Well, if you are saying highly-ranked and authoritative sites, they don't only post unique, relevant, and keyword rich contents. In fact, if you will notice and look closely, they completely ignore keyword rich contents. Why? Because so-called seo contents now are proven to be the most spammy ones compared to a content that is written with pure information, relevant images, and providing what users need.
Content is king. But it will only become a king if you are able to make people impressed and satisfied with your content.