Their argument is that, by defining a robust process, software manufacturing can be controlled , and predictable results can be obtained. By requirement, a good manufacturing process is repeatable. To answer this question, look at what motivated design of processes in the case of manufacturing industries - repeatability and mass production. As a result, we can build cars in shop-floors and not in laboratories. This is not necessarily true in the case of software.
Software development is still largely a laboratory exercise.
We can not completely develop software in assembly line production houses. At least, for the present. While software engineering processes can guarantee that certain steps are undertaken while developing the software, they can not bring in predictable quality attributes. Any application development technology or product or specification abstracts a significant amount of the underlying infrastructure. This shields application developers from the infrastructure issues.