...but we are expected to know that we don't know :-)
We are the sum of our experiences and flavoured by our natures. In the business world, the more experience one has, the more it is assumed we will make less mistakes.
The problem is society and technology are changing so rapidly that, even if a situation feels similar, the nuances may be brand new. Past experiences may have no firm anchor in a current dilemma or decision. So assumptions need to be reconfirmed every time.
For some, this kind of uncertainty makes new projects feel like mountain-climbing every time. For others, it keeps planning fresh.
A recent HR blog posting on Harvard Business Review talks about how one HR professional loves to read "unconventional" resumes as potential employees who are adaptable to change can be identified if they also show change as an acceptable course of action in their own careers/lives.
The point that many seem to be making these days is that we don't have to know everything; we simply have to be willing to learn new pieces.