It really isn't though. Most sources are not mentioned in a typical article. Prominent interviewees are mentioned by name, but more often then not information comes from a quick Google search, someone not established enough to be mentioned (eg you would mention a professor, but not a young research assistant/secretary), or even Reddit.
Not to mention that a large amount of articles contains no original research at all. Often it's a press release, a rewrite of a press agency release, or just a copy/paste and rewrite of some other website.
Sure, and this info should be attributed to the reporter, who gets full blame when it's wrong. Though I do agree it'd be nice (in a world where the First Amendment didn't exist) if there were a rule banning such weasel phrasing such as "Some say that" and "according to several experts [who are never named or specifically referred to]"
As for articles being a rewrite of press releases or other articles; yes, that's obviously an issue, and one that many journalists get irate about. But even in most of those cases, the sourcing is clear: "[Regurgitated rewrite of news lede], as first reported by the New York Times [link to original article]".
But for the archetypical article containing original reporting, here's an example of how a source involved in an article might not get listed:
Original article: https://www.nytimes.com/2019/11/03/business/drunk-driving-br...
Behind-the-scenes writeup that acknowledges an expert who isn't named in the article: https://www.nytimes.com/2019/11/03/reader-center/breathalyze...
The reason why the expert wasn't named, even though he was very helpful throughout the reporting process, was because the article's ultimately relied on the content of original documents. The expert helped lead the reporters to those documents, but his expert opinion was not needed and thus not cited for the article's authoritative conclusions.
It's true that reporters often won't credit where they first got a tip or where they saw something. But if they do their own parallel reconstruction – e.g. they first read an anecdote from a viral tweet/Reddit thread, but then they track down the original source themselves – as a reader, I ultimately care who the reporter and their editor explicitly say they vetted/vouch for, when it comes to the published version of the info.
This is something that is ostensibly already in a typical article. Or are you suggesting that the reporter list everything that was researched, and every one who was talked to, regardless if it made it into the published version of the article?