A cook who can't cook is not a good driver. What are the similar metaphors?

1. A painter who can't draw is not a good writer.

2. A calligrapher who can't write is not a good teacher.

3. Players who can't swim are not good bosses.

A tailor who can't make clothes is not a good tour guide.

An author who can't write a novel is not a good bartender.

6. Programmers who can't repair computers are not good foremen.

7. A teacher who can't teach is not a good cleaner.