Website optimization covers around 30% of Google’s ranking factors; however, off-site factors make up the remaining 70%, and link building comprises the bulk of it. Many marketers ignore link building, and then they wonder why their clients aren’t ranking in the top search results.
Why are backlinks so important to Google? It’s very easy to stuff keywords on your website and try to manipulate Google. However, receiving links from 3rd party sites are very hard to game. Since it's difficult to control activities on other websites, Google places a huge priority on backlinks. Plus, the Google search engine is built to retrieve information based on cross-references and associations (links).