Because that's how they've done it since 1843. So there.
> The debate over the [US] Constitution ... was done almost entirely through pseudonyms. Most writers, on both sides of the question, chose to conceal their names from the public, as was the custom in the late eighteenth century. A free press, one of the most essential principles for the founding generation, was presumed to include a right to anonymity.
http://www.learnliberty.org/blog/anonymity-and-doxing-in-the...
I mean, the Federalist Papers "fifty-one!" sequence would probably have made a better refernce, but they're not too explicit about the pseudonyms there.
Prior to the introduction of moveable-type printing to Europe, ~1440, and excepting the Classics (Greek and Roman works), authorship and authority were not particularly highly considered. Without the means to cheaply reproduce and sell works, authorship wasn't a key concern. There were ~30,000 volumes (as in books, total, not merely titles) in all of Europe in 1400. By 1500, 20 million, and approaching 1 billion by 1800. Literacy and costs (in wage hours) also changed tremendously, rising (from ~5-10% to 90%+) and falling (from ~$0.5 to $1 million modern equivalent, to perhaps a few dollars for a work) as well.
"Moral rights" of authorship are a French tradition, though I'm not certain of the date of origin. Modern copyright law dates to 1709 and the Statute of Anne.