Because topic areas are in no way equivalent. My expertise is specific and not of broad public interest. But tax policy, for example, absolutely is of public interest. And news organisations have been covering for years. Individual journalists have been covering it for years and are as much experts as anyone out there.
Let’s take Syria for example. Do you think the reporters reporting on Syria and the complexity of what is going on there are experts in foreign policy? I don’t. I don’t think government affairs reporters are experts on tax policy either. I think they interview people and create a narrative and write said narrative. They are not usually experts on what they write about. They are writers crafting a story for an audience. I generally think they try their best but they get it wrong due to pressures of the job and limited time/ability to vet things. Especially these days with dwindling budgets for news organizations.