Most physicians are trained to think of IBS as stress induced or as a type of psychosomatic disorder.
There has not been an easy medication cure for IBS, therefore it has been framed in a way that suggests that it is more your problem than the physicians. Although some cases of IBS are no doubt related to mental or emotional issues, and stress and anxiety can aggravate IBS (as well as most other medical conditions), they are not the predominant causes of IBS. More often, IBS causes you stress rather than the other way around.
Medicine has a long history of blaming medical conditions with no straightforward, obvious organic cause on some form of psychological disturbance, such as stress – only to discover later the organic cause of the problem.
One excellent recent example of this is stomach ulcers. While stomach ulcers were once thought to be a stress-induced disorder, it is now well accepted that the bacteria Helicobacter pylori cause them, and that they are treatable. Interestingly H. pylori can also cause IBS symptoms, and now that we can test for it and treat it, there is one more reason to believe that stress is too often blamed.
It is true that stress can make the symptoms of IBS worse, and that your digestive system depends on relaxation to function properly. Occasionally stress is the sole cause of IBS. But far more often it is due to something else. If you still have symptoms when you know that there isn’t much stress, then there is more to the problem than stress!