There is a certain level of shortsightedness that we can have at times when it comes to our expectations about who can be a good instructor, and what they would be capable of teaching. There is a false bias toward believing that those who excelled at a particular task would be the best to teach it to others. We also question the ability of one who performs one task to be able to educate on another.
This tends to be a particular bias when it comes to position coaches in the NFL. A lot of position coaches were never stars at the position that they end up coaching, though many, likely the majority, at least have a background in that area.