Weird Erotica?

You covered the questions pretty well, Mysterio, but I'll throw in a few more thoughts.

Why do I never see this stuff anywhere else?

Anyone who hasn't seen this stuff anywhere else just hasn't looked in the right places, or hasn't noticed macrophilia when it's right in front of them. http://www.gts2.net/links/index.php has a lot of links, but that's hardly a complete list. And think about all the B movies (and a few A movies, like King Kong) that feature giant beings.

Is it something about Writing.com that attracts people interested in these kind of things?

I think the attraction of Writing.Com for macrophiles is that it's a place where anybody can sign up for an account and post writing at no cost, and word of mouth has helped spread the word. Writing.Com is even included on the page page I just linked, at the very bottom.

I mean, almost all the Interactive Stories are exactly the same.

It's unfortunate that many of the stories are exactly the same. It's because they're written solely for the purpose of sexual arousal, without any regard to developing characters.

By the by, I realize it probably isn't nice of me to call this stuff weird. Don't be offended it's just a forum post.

While I don't care whether you think macrophilia is weird or not, I don't agree that it's weird, relatively speaking. In my case, it's an attraction to women who just happen to be magnified in size (in terms of height, not girth). When you compare that to things like eproctophilia, pyrophilia, and emetophilia (sexual attraction to flatulence, fire, and vomit, respectively), my desires seem pitifully mundane.

It's all a matter of perspective.