A BPMN model just gives you some information about the process: who (or what computer) needs to do what, and when. So the BPMN model helps to orchestrate the work. But that is not the whole story. If you look closely at a BPMN model, you will see it does not hold all information you would need to automate a process. For example, what is the exact thing a user needs to do when getting a user task appointed? That's not in the model. Or, what is the exact decision logic to use when hitting a Business Rule task?