Some excellent comments in the last few posts. To address a few.
1. On the idea that scholars in the humanities don't know how to work together: definitely not!! I myself am working on a collaborative project (a translation of Duns Scotus) with someone in the US. I have never met this person, but we have worked together by email for more than two years on a large project. I have never had any trouble working with him, apart occasional complaints about the speed of progress. We have met many substantive issues about which there is disagreement and potential conflict. In every case we have met this by reasoned argument, evidence, sourcing and so on. Occasionally we have had resort to third party views, but only when we agreed the problem was too difficult for us to resolve. And the third parties were taken from our small community of co-workers, which is evidence in itself that we are a community, used to working together. As further evidence, consider the great translation of St Thomas's Summa Theologiae into English in the 1930s. A huge undertaking, made by many translators working together in harmony. A sample here http://www.newadvent.org/summa/1001.htm . So, humanities scholars often work together, often in large groups.
2. More pertinent is Michael's comment "With mythology or other humanities subjects, the academic paper may rely on some facts or things that are very nearly facts, assumptions that have universal acceptance though they may not be provable in the usual scientific sense." Again, humanities scholars have worked out methods of dealing with the problem that 'facts' in the humanities aren't as clear-cutand decidable as facts in the hard sciences. Whereas any component mathematician knows what Cantor's or Goedel's proof is, to state the key ideas of Aristotle in a paragraph is much harder. But we have worked out ways of dealing with this, and that is part of every student's training. Indeed the idea of 'primary' vs 'secondary' source comes not from the hard sciences, but from the humanities (from textual criticism, I think, but I am not sure). As an example of this, consider the well-known distinction between Anglo-American 'analytic' philosophy and 'Continental' philosophy. This is profound, yet because we all have a common humanities background, we are able to work together on Wikipedia. There are about 4 academically trained editors working on Philosophy in Wikipedia, 2 from the Anglo background, 2 from the Continental. I have never known any disagreement among this group. The big disagreements come about in the edit wars between the academically trained philosophers and the untrained ones, for example the 'Objectivists' or disciples of Ayn Rand. These have had no formal training in the way of resolving disputes and the way of resolving difficult problems that is essential to academic training.
In summary, the problem is not that humanities scholars cannot work together. Many of them do, sometimes in large groups. And the problem is not that there is no method of working problems out. Such methods are integral to training in the humanities. (Indeed, that is the main component of such training - it's not the content of what you learn that is important, it is the well-defined methods of approaching problems). In my experience the problem of humanities in Wikipedia is that the methods and training of the 'experts' is so fundamentally different from that of 'Wikipedians' (who by and large have no training at all) that disputes nearly always turn ugly.