I see what you mean. However, given the large number of different CMSes, I'm not sure designers would appreciate having to re-learn how to design every time they use a new CMS. Right now they use a relatively small "default" set of tools, with PhotoShop being the chief one.
I'm not a designer, but my feeling here is that designers wouldn't appreciate the workflow you're suggesting. Any designers here who'd like to weigh in?