Keep in mind, though, that we are told that SBURB chooses aspects that will challenge the players.
It's entirely plausible that the challenge for Seers is to eventually come to terms with the fact that they can't control everything. Look at Terezi, who starts out being the most manipulative player of the Troll session, and probably strongly believed that she could control anything and everything. She even learns to control herself to the point that she can use her other senses to surpass her destroyed sense of sight. But in the end, she is unable to stop Vriska without killing her. She is simply powerless in that situation, and there's nothing she can do to avoid being powerless.
Sollux starts out with some pretty clear feelings of being absolutely powerless, and we can definitely see why. He can only barely control his own mind, and loses control like it's going out of style. In the end, I don't think Sollux ever really got past that mental block. The mage, therefor, could very well have the ability to change the cogs of fate, to avoid these inevitable things, and the power is therefor given to the most fatalistic and "powerless" individual within a session.
Above all else, SBURB is a game about growing and maturing. Each player has a tailor-made experience that will help them grow the most. I've heard and read and seen it said that the trolls probably never completed any of their personal quests. Even Vriska, who was by far the most powerful player in their session, almost certainly never completed her personal quest. The only one I think might have is Aradia, and I really have no evidence for that.