Here's something that not many people will tell you: google hates people who do SEO (who buy backlinks, more specifically). So, they (google) do whatever they can to stop people from doing that, and one of the measures that google had put in place to make life hard for people who buy backlinks is by making the easy-to-get backlinks useless.
In other words, the easier a backlink is to acquire, especially if it's a backlink from free websites (bookmarking sites, web 2.0 websites, blog comments, article directories, profiles on authority websites like microsoft, spotify, or whatever) the less value those backlinks have.