There are important points to think about here. Staying calm in the face of some else's anger or personal attack takes courage and self-control. It's certainly not easy, but it is a choice. (Sometimes so is walking away until you can deal more calmly with the situation. Sometimes though, that's just not possible.)
You are absolutely right. It takes a lot of courage and self-control and it's certainly not easy, that is why a lot of people lost control of their behavior, especially working in a stressful environment.