I think it's because SO many startups (at least the one's that get a lot of press) are related to social networking social bullshit. Contrast that with "world-changing" endeavors, and the difference is pretty stark.
I think we’d see a pretty wide range of opinions if we polled folks here as to which of the current crop of both established companies & startups they felt were companies that really, truly “changed the world" or were in the process of doing so.
Look at this way though.
Let's say you share a picture of me at a concert and my dream romantic partner sees it, decides she must be with me, and tracks me down. We fall in love and get married and I never again have to think about dating, because I choose not to.
Now I have more mental resources dedicated to the tasks of providing everyone with healthy food, disseminating compassion, ...