If there is a novel inside every English teacher, why do they sometimes suck?

Obviously, many English teachers have become successful upon publishing their creative writing (Stephen King, Rick Riordan, et al). But there have been a few times where I have read literary fiction from English educators up to the college level whose writing plain sucked. I wonder why someone who knows so well the complexities and necessities of good writing, who is able to provide insight so well to others’ writing, and devotes their whole life to the subject, holds little merit to their own published works.

If there is a novel inside every English teacher, why do they sometimes suck?
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