In fact, design goes backward and forward whatever the starting point. As you point out, it's not that curriculum designers don't think of goals when designing a syllabus. It's just that a content item is not expressed as a goal. Similarly, a central design has a broad outcome in mind, even though it might not be specified in detail. A backward design will often take account of the process of teaching an item in formulating the outcome and it will often have content built into it. All three may be thought of the same time, rather than being linear.