Actually this gave me an interesting thought. The amount of effort required to "share" or "retweet" something is almost zero - it's a click. On the other hand, the effort to summarize something in your own words is more. Not a whole lot more, but enough that many fewer people do it. So I guess the tl;dr here is: maybe the shares that required more effort could be ranked higher.

Maybe a better summary: "retweets are cheap"