4. Sully's Arc Is Already Complete
Speaking about the thought process behind having Chloe and Nadine lead The Lost Legacy, Shaun Escayg said: "Even before we settled on this particular story we were exploring Sullivan, we were exploring Cutter, and pairing each other up, thinking what would be right."
As a father figure, Sully had completed his mission. He had mentored the street rat he adopted from Colombia, witnessed the child evolve into an adult, and he had become a granddad.
Shockingly, he even announced that he quit smoking.
An exploration into Sully's past is not necessary. Only an explanation of how Drake became Sully's prodigy is needed, and that was established at the beginning of Drake's Deception.
Sully's behaviour, his quips and his appearance, has already informed players about his youth. He was a Casanova who bedded numerous beauties, he was a smooth, easy-on-the-eyes, driven by money explorer. Plus, he smoked worse than a chimney.
Additionally, no revelation about Sully's past would change the father-like man he ended up becoming.
Having Sully seek treasure on his own in the present would also not be enticing because Sully's role is to be a protector. As Drake's dad, it is his responsibility to put on his red Havana shirt, comb his moustache, and pack a bunch of cigarettes, so that he can fly to some godforsaken part of the world to once again bail out the man who had become his son.
Without the father and son dynamic, Sully is not necessary. This is why he frequently disappeared when Nate always joins up with Elena.