Because, you know, bones are organic.
Actually, MoD posted their thoughts on this subject recently, and I very impressed with the core idea. Essentially, whatever idea you start with is the idea you explore to its gruesome end. That’s outstanding advice, though I can say from experience that it’s not going to work in every instance. I’ve written stories that flow from and orbit a central idea–and they tend to be hard to publish. It’s a good approach for MoD and other themed anthologies, because they give you a very good idea of what they’re looking for to begin with, and you know that your idea (if you want them to buy it) has to start with a germ that looks something like (Their Theme + My Idea). The recently closed Zombiality II anthology, (closed market) for instance, was looking for LGBT Issues in combination with the Zombie Apocalypse trope. It’s easy to take those two elements, combine them with a single idea of your own, and then produce a piece of short fiction that will at least interest the editors.
But when you’re just writing stories because you want to express your own creativity, it can be really hard to find a market for the results. Some publications claim to want any “compelling” writing, but at the end of the day, this is because the editors don’t want to limit their publications, and possible because they just don’t know what they’re in the mood for until they read it.
So if you start with the idea, “An eyeball is growing from my arm,” you might not be able to find a market for the result anywhere. This is all the more reason for professional fiction writers to study their contemporaries, particularly focusing on what the editors (and readers, if you’re in a position to look at fan mail) seem to want.
That’s not to say that there is no solution to this. It’s to say that if you’re writing for people other than yourself, you should actively try to do this. It’s OK (and probably healthy) to mix our ideas with the yearnings of other people. So the editors at Super Elf Whammo! magazine want light-hearted, goofy stories about the hijinx of lithe woodland pacifists. The idea of an eyeball growing out of an arm can fit very well here (or anywhere), even as a main premise. You just have to be willing to dilute your ideas with other people’s.
I think people call this “selling out,” which is actually code for, “I’m jealous because you make money doing this and I can’t.”