Sorry, the reason for the stereotype is the fact that fanfiction is findable only on unmoderated internet archives where anyone can post. If you had to look on the internet for all your original fiction, you'd have the same problem. Also, it's in some ways harder to use someone else's voice and be bound by characters that maybe have traits you're scared to write about than to be able to write in your own voice and avoid certain kinds of characters.

But when you compare cherry-picked original fiction weeded through by editors until you get to read only a fraction of the total submitted for consideration and utterly unmoderated, undifferentiated fanfiction by good and bad authors alike side-by-side in the same archive, of course the original fic is going to be better.