Because it's simply a useless but provocative truism. We all agree that the best practitioners are often not the best teachers, we all agree that the best teachers are often not the best practitioners, and we all agree that some of the best practitioners do in fact teach.
So repeating the old cliché, "those that can't, teach," even with qualifiers, serves as nothing but a dig at teachers in general—a group that gets more than its fair share of digs these days from a lot of sources.
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