One problem with Wikipedia is that editors often know little or nothing about the articles they edit. So even if, ceteris paribus, the collective judgment is superior to an individual judgment, a collective judgment from the completely clueless will not be superior to a judgment from an expert. What one needs is a collective judgment from experts or near-experts.
This is not such a problem on articles that attract little popular attention and that require some relatively rare technical competence for an editor to make any contribution. Articles in mathematics and statistics, and in the hard sciences, generally have few editors, and these are generally quite knowledgeable. But articles on current events, on pop culture, on the softer sciences and history--these articles attract the modal Wikipedia editor, who feels free to make any "improvement" and move on.
Who is the modal Wikipedia editor? Someone who finds life on Wikipedia more fulfilling than life in the real world: someone for whom anonymity and egalitarianism offers an increase in status, someone for whom interaction with other Wikipedians offers an increase in the quality of human relationships--someone whose real life is characterized by subordination and loneliness. The modal Wikipedian is a teenager, alone in his room after school.
To most academics Wikipedia doesn't look like an attractive place to "publish." Journal referees may often make stupid comments, but they are vastly more knowledgeable than the swarm of random editors that snipe at articles in Wikipedia. Dealing with other editors feels a bit like dealing with students, but with the big difference that students are almost always respectful, while Wikipedians, even the really dumb ones (maybe especially the dumb ones), all think they are just as good as anyone else. Anonymity and egalitarianism, for academics, means a loss in status; it is only those with no reputation and of low status who will see status improvement in Wikipedia.
Thus only those with no reputation and low status will have an incentive to participate heavily in Wikipedia. Teenagers, because of their unfortunate position in the life cycle, fall into this group. And teenagers, because of their youth, lack both the experience and the education to have near-expert knowledge in any domain (except, perhaps, popular culture).