The level of terrible writing everywhere is just too gorram high. It's just that a high percentage of the "professional" writing we read on a regular basis goes through an elaborate screening process that filters out most of the Mary-Sues and horrible writing. (Most. Not all. *coughTwilightcough*) Fanfiction, on the other hand, has no screening process. We're exposed to every single piece of crud a writer has the urge to "publish".