Because there are A LOT of copyright and intellectual property laws that prevent you from "just" writing fanfic and parody. If I were to write a fanfic based on, say, Buffy the Vampire Slayer and sent it out to agents and publishers, I can guarantee you it would be rejected outright without being read. Nobody would want to face the wraith of Joss Whedon and his lawyers. Hell, you might even get some personalized rejection letters that basically read, "ARE YOU $#&^ING INSANE?!" You might get away with it for a couple months if you decide to self-pub, but many outlets would probably drop you when they find out about your copyright infringement. About the only ways you'd be safe are if you're chosen by a property holder to write an authorized novelization (see also: Star Trek, Star Wars, et. al.) or through Amazon's Kindle Worlds, which has licensed properties available for you to write and publish officially sanctioned work (which really isn't the cash cow Amazon was wishing/hoping for, but that's neither here nor there).
Same for parody. It's not exactly sue-proof, either. Just look up what happened with The Wind Done Gone, a parody of Gone with the Wind. In a nutshell: lawsuit filed by Margaret Mitchell's estate, long court fight, book finally released with a note on the cover it's unauthorized by Mitchell's estate.
Maybe, just maybe, you can find something that's actually in public domain. You can write fanfic and parody it until the cows come home (see also: Pride and Prejudice and Zombies and Sense and Sensibility and Sea Monsters) without fear of repercussion and scary lawyer visits. Copyright and intellectual property law are so deep and complex, you really need to consult a lawyer (which I am not, this is just for basic information purposes only, standard disclaimers apply) if you want to "just" venture out into trying to publish fanfic and parody.