A check pilot will tell you what you are doing wrong. An instructor will tell you what you are doing wrong and show you how to do it right.
There is a lot of psychology involved with instructing. Military instructors have many weeks being trained in basic physiology. Nobody performs well under pressure, so why put a person under pressure? If you want to get the best out of a student, F/O or anyone else for that matter, then let them be in a relaxed environment!
If you are instructing and you are yelling and screaming, then you have the problem and you have lost the plot.
You could be the "best" pilot in the world but if you can't put it across then you aren't the person the training organisation requires.
In the civilian flying club environment there is a commercial aspect that has to be considered also. The student is the customer! The student is keeping the instructor in a job! So bad instructors need to be rooted out from a business point of view as well.
As has been said it is not easy to train a good instructor as they usually need to be "born" with that ability.