Because, despite what some lawyers might like most of us believe, it's not actually illegal, counter to good sense, or constituting practising law without a license for creators and users of copyrighted materials to share their knowledge about copyright and how it impacts their works and uses of such works.
Publishing is on topic, and copyright (and trademarks and patents) is an integral part of RPG publishing and licensing. Since publishing is explicitly on topic, that aspect of publishing is part of our topic.
That's really all there is to why we host such questions. You cannot be a content creator in this day and age without familiarity with copyright issues, and due to the vigorous creative reuse of materials, it's hard to even be a mere user of RPG materials without being exposed to copyright issues (especially in the last 15 years, with the rise of the OGL and the OSR).
The questions are almost always far too broad. 'Is this a violation of copyright?' Where? In what country? Under what law?
Almost never too broad. Copyright laws are widely harmonised (for better or worse) among Berne Convention countries. Being the Internet, it's also nearly impossible for a Berne Convention signatory to not be involved in any given RPG publishing situation. So, it's actually usually pretty clear what copyright regime is relevant to a question about copyright.
Every single one of the copyright questions could be re-posted, swapping a role playing property for some other media, and get the same answers.
To the contrary, our answers are almost always extremely pragmatic—that is, they are focused on the particular issues that a real RPG publisher has to be concerned about, which are not universal and are particular to the RPG publishing environment.
We are literally giving legal advice without being lawyers. I am not a lawyer, but I understand that it's probably not a good idea for us to be doing that.
This is a widespread myth. The only way to violate the prohibition against practicing law without a license is to mislead your audience into believing you are a lawyer. We haven't had a problem with that, and our community moderation tools are sufficient to the task, in the unlikely case that we ever do have that problem.