People who already know how to do something tend to be no longer aware of all the little things they learned as they were developing that capability.

On the other hand, people with lots of experience teaching newbies often have quite a detailed knowledge of the most common failure modes for beginners, and can be very useful for precisely this reason. As a second best, people who have only recently learned how to do things often remember their own learning experience sufficiently well to be useful (provided there aren't so many distinct failure modes that their individual experience is unlikely to be representative).