Garriott did this because he wanted the real player to be responsible for the character's in game actions due to the ethical parables he designed into the story.
Only if you were playing "yourself" Garriott felt, could you be judged based on your character's actions.
The use of the term avatar for the on-screen representation of the user was coined in 1985 by Richard Garriott for the computer game Ultima IV: Quest of the Avatar.
In this game, Garriott desired the player's character to be his earth self manifested into the virtual world.
In computing, an avatar is the graphical representation of the user or the user's alter ego or character.