There are possible logical explanations for Tav to be selected as leader, I had mentioned one - she/he just has the better parasite. Like an alpha parasite. or maybe its like the Nameless One Planescape: Torment - he attracts people like himself, who exist in opposition to their natures. All of us having the parasite draws us together, and we subconsciously select a leader.
Nah, not likely since you can be an origin character and be a leader of a party.
My bet is no one really thought about why custom character is a leader, since the focus was and still is on origin characters.
