Why are so many of the submissions Wikipedia articles, which usually already have a page in the translated language?
Primarily because users don't bother to check (or some just don't care) before submitting documents, and also because DL doesn't have the capability to curate user submissions. Beyond Wiki articles, I've seen Project Gutenberg books submitted that already have English translations available. I wouldn't be surprised to see documents that were originally written in English, and then translated to another language submitted back to DL.
Luis has suggested reporting such documents as inappropriate so they can remove them. I've begun doing that with documents I encounter.
Because they're not under copyright, and we don't have a general way of dealing with copyrighted articles. Hopefully people translate stuff that isn't the same as the English Wikipedia article.
Dansdaci is right. Aside from teaching people languages, Duolingo's main purpose is to help translate the web in order to enable speakers of all kinds of languages to a wonderful diversity of content that would otherwise be inaccessible to them. If an article is already translated, then nothing is being accomplished, because everyone who speaks French and English, for example, can understand the content.
Don't even get me started with examples of this in immersion, like the Wikipedia article for eye in the German immersion: https://de.wikipedia.org/wiki/Auge
Don't you think the English version would have the same amount of information, if not far more than the German version?!?!
I'm glad more attention is being paid to this issue.
mstreeter had this to say about the future of Wikipedia translation: "we plan to publish the translations to the target-language Wikipedias, but aren't doing so currently. You're welcome to do it yourself in the meantime."