Why do people make their work private?

[Cosimo )O(]

I assume you mean people's activity on Answers, rather than their poetic oeuvre outside Answers.

There is a very good reason to make ones activity private - to stop random trolls and other users who may resent you (for whatever reasons imaginary or real) trawling through your activities and reporting them for abuse, thus destroying your account.

It takes a brave user, or a nonchalant user, to have activities open to the view of all on Answers.

How do newbies discover you? They add you to their network - they become your fan and hope you reciprocate.
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However, if you're talking about poets keeping their oeuvre to themselves, that is because ones poetry is intensely personal. It would be like posting your biography online, only more so. It may seem strange in today's twittering world that anyone would refrain from baring their soul in public. But some people are still quite private.