Place an ad for a COBOL developer and an ad for a journalist with knowledge of physics, and see which one you fill first.
90% of journalists are generalists. That's just how journalism school works, since the reporters don't know where they'll end up working.
Most of the remainder are people who were experts or have degrees in a particular field, and ended up becoming journalists.
That said, the second group are often the best at their job. I know journalists who were in the psychiatry, IT, health, and other fields before they started reporting. They all excel at their craft.