I think Wikipedia is a successful collaborative site because there's much less room for people to argue over simple, declarative text than any other medium (though of course they do argue, all the time). The arguments that could be raised over the editing of a collaborative documentary are much less cut and dry.
Also, the density of something like video make it much harder to review changes. If someone moves around a few scenes and in the process splices in still frames of porn, how is anyone going to know without carefully viewing every frame of changed video? Short of someone creating a very sophisticated diff viewer for video, that would have to happen for every single change that's committed. It would become much harder to differentiate between real changes and blatant vandalism, which would inevitably drag down the quality of the videos produced.
Surely rather the "other fields" of collaboration are things that orbit the blackhole known as the free market. (wait, can you actually orbit a blackhole...?)
Also, the density of something like video make it much harder to review changes. If someone moves around a few scenes and in the process splices in still frames of porn, how is anyone going to know without carefully viewing every frame of changed video? Short of someone creating a very sophisticated diff viewer for video, that would have to happen for every single change that's committed. It would become much harder to differentiate between real changes and blatant vandalism, which would inevitably drag down the quality of the videos produced.