I think one of the main reasons may be that a lot of what appears to be journalism actually isn't - it's press releases from various companies and organisations, known as "churnalism". It's surprisingly easy to find churnalism, even in what appear to be fairly authoritative news sources (see churnalism.com). If journalists routinely cited their sources they would have to admit that often large sections have been copied verbatim from the public relations output of companies.
But the point of the article was that in at least one of the three stories mentioned, the journalists appear not to have even read the press release for the study in question.
