I'm much happier proofreading others' things than proofreading my own (see, Hues of Infection, lol).
Proofreading for contests can be iffy if there isn't enough time. There's also always the concern of being wrong in the proofreading or ruining the style by imposing my own over it. Pretty sure the only story I proofread (that I didn't write for) that got published has since been removed, but I like to think my effort kept it from being unpublished right away, heh.
Maybe Bezro will take the communal approach by using the sneak preview toggle. But it is true that most people seem to be writing (or procrastinating) their own submissions.