A person can be an expert in a field but not be able to teach it. Often times they can't pass on their knowledge in a manner that the students can absorb. Certified teachers have expertise in how to teach what they know. (Needless to say, not all of them really have the expertise that they were trained to have. )
To add to your excellent post, any good teacher is forever a good student, and most will probably learn more in their first couple of years teaching than they learned in formal 'education' schooling.
