Engineering usually follows certain established disciplines, methodologies, and workflow, which is repeatable and process driven. With a mature process in place, the difference an engineer make can be minimized, which is good for production purpose, and is also the motivation for the popular CMMI adoption.
Architecture is a creative art for operation; it is more conceptual oriented, with emphasis on vision, insight, and creativity. Therefore, the difference made by architects can be enormous, while engineering is the next step to follow. I am not ignoring that the architecture creation/development process also need discipline and skills that can be trained to support the talent and creativity that generate ideas and concepts.