Because journalism is a systematically dishonest profession populated largely by people who do not want their readers to be able to catch their mistakes and do not want to give credit to the researchers who made the discovery that earned the clicks for the news article. They care about entertaining, not informing; the truth or falsehood of what they are writing is irrelevant to them, except as a constraint that might impede the telling of a sensational story (all else being equal, a story is more sensational if it is true than if it is fiction). Like PageRank-hungry SEO scum, they will only name sources if they think some of that the source's credibility will rub off on their article, never vice versa.