Expertise tends to be strongest in situations in which your environment sends you a regular series of predictable events with immediate feedback and clear outcomes. Physicians, athletes, firemen, and chess masters can build good expertise in their fields because they deal with fairly rule-based systems with strong feedback and lots of repeat experiences.
Some professions are not so lucky. Stockbrokers work in an environment with good feedback, but poor reliability -- faced with almost the same situation on two different days, they might take the same action and get completely different outcomes. In this environment increased experience does not lead to increased skill. In contrast to the earlier example of physicians, radiologists and clinical psychologists have much weaker feedback, getting little or no direct feedback on long-term results.
Occasionally, you might find what psychologist Robin Hogarth calls a wicked learning environment. This is a setting in which feedback is so corrupted that it causes additional experience to lead to worse performance. His classic example is a doctor in the 1900s who believed that he could detect the early stages of typhoid with great efficiency; when he thought that he detected signs of typhoid he would complete an examination that included palpating the patients' tongue. As he was a carrier of typhoid himself, he was invariably correct when he came to the conclusion that the patient was about to come down with typhoid.
Political pundits are in a similar situation -- they may have to wait weeks, months, or years to find if their predictions are correct, and get very little feedback from their viewers that would discourage incorrect predictions. The feedback they do get is from ratings, which are tied to making new, interesting predictions now, and do not encourage looking back at old predictions or focusing on the possibility of moderate and predictable outcomes. In this environment personality and self-confidence are more important than expertise.
It is also possible to set up a wicked learning environment for yourself. Lyndon Johnson was not a military minded man, and when faced with the war in Vietnam he dismissed three of his four military aides and requested that the military speak to him only through defence secretary Robert McNamara -- a request that McNamara happily complied with, the two men building a chain of command that carefully filtered out any disagreement and surrounded the president with yes men. The results were a messy, protracted war that did not benefit the United States, and was not particularly unique in its cause when compared to many other bad political decisions throughout history.
The opposite of a wicked learning environment is a kind learning environment: one with clear feedback, frequent feedback, and a known range of outcomes. While we are not often charged with designing learning environments, it is important to consider these factors if you happen to be a product designer, manager, professor, teacher, or especially a parent.