You don't really know your own preferences until you actually have to make the decision.

You've made a lot of decisions in the past to calibrate from, so you develop a pretty good intuition for your preferences.

But an assistant (a person, or a computer) only has limited examples to draw from and calibrate from.

This is one of the reasons that "partial executive assistant" services and computer based assistants are often underwhelming, and hard to hand off meaningful work to.