Because the occasional self-published book does get picked up by a traditional publisher. But, well, the (not-so) occasional submitted manuscript also gets picked up by a traditional publisher. (Publishers Weekly and Publishers Lunch and the like still report new deals every week, after all.)
Yet a self-published author getting picked up by a traditional publisher is a news story and a sign of how Everything Is Changing, while an author who submits their manuscript and sells it without self-publishing first–even though pretty much every day someone in New York is buying a traditionally submitted manuscript–is not only not worthy of mention, but is so not worthy of mention that more and more people are telling new writers that they shouldn’t even try to sell a book this way, because that sort of sale just doesn’t happen anymore.
This troubles me not only because it discourages writers for whom traditional publishing is the best route, but also because it makes it hard to see clearly those cases where self-publishing is the right path to take, and to make smart decisions about ways of pursuing it.