How come regurgitating the author is called 'realism' in original fiction and 'Mary-Sueing' in fanfiction?

Tavia

Because in original fiction, the author's avatar is a nuerotic with memories of an alchoholic father, demanding mother, who has worked their way out of (poverty/an abusive home/a series of deadend jobs) and is reaching a crisis stage in their personal life while battling a serious illness. In fanfic, Mary Sue is trained in piloting/espionage, has perfect hair and teeth, can outsmart Servalan without setting down her teacup, and charms even the coldest hearts. I imagine a short, acne-prone, angst-ridden avatar who gets left behind when their teleport bracelet breaks and the ship is in danger wouldn't get too many complaints. I have come across plenty of these plastically perfect people in a Star Trek fan club I was part of, where everyone was encouraged to create original characters to be used in fics. Unfortunately, one woman specialized in what I called 'Federation Yuppies from hell'- everyone was a gold medalist in upper-crust sport of choice, graduated top of their class in their division, etc.