The point that Fred makes sounds good, and I'm tempted to agree. In my dozen years of adjuncting at two-year and four-year institutions, I've met teachers of writing who actively disliked writing even intradepartmental memos and meeting minutes. The community colleges where I've worked didn't expect any more writing of their faculty than texts of that type.
But I can't agree that lack of writing on the part of writing teachers has to do with how writing is taught. I'm troubled by the examples that come to mind of writers who teach developmental writing students processes and rules that have little to do with their own writing experience. A fellow adjunct, a poet, requires that thesis and topic sentences be indicated with asterisks in students' final essays, and he teaches that a certain amount of their reading-based writing should be quotation on a basis something like "one or two quotations per paragraph." (I see his students' work in our departmental review of student portfolios.) Cases like this make me wonder what happens between teachers' own experience of writing and their classroom practices.
One possible explanation is the belief that students are different from us, a belief I've heard voiced over and over in the community college, where the gulf between basic writers' experience and teachers' experience is usually very great. My own teaching practices grow in part out of what I know of myself as a writer, but when I have voiced this premise in meetings, the response has been, quite literally, "But they're [the students are] not like us." To those who view students in such a way, a pedagogy based in the teacher's personal knowledge of writing must seem irrelevant, and hence rule-based, rigid structures are substituted.