I see errors all the time in mainstream media. Sometimes these appear from some kind of info file that they raid every time they have to look up a subject, so the same information is quoted again and again (even if inaccurate). A lot of things in life are subjective and open to interpretation, especially when it comes to politics and culture.

Mainstream != serious. In fact it's quite the opposite, as serious news organizations cannot match the output of mainstream news. Even one story per month is a success for many.

In serious news organizations, there's quite a few steps between a journalist writing a draft and that draft being published. Fact-checking is one of them, having a competent "boss" (called an editor) is another.

Most news orgs have both a "serious" department and a "publish as much as possible" department, with far different requirements. In general, if you're publishing something along the lines of "X said Y", you don't need a rigorous process. If you're doing an investigation in which you're accusing someone of doing something illegal, then you need a far more rigorous process, otherwise you'd be sued out of existence pretty quickly.

Of course, having a rigorous process doesn't mean you won't get sued at all, but there's a term for that: SLAPP (strategic lawsuit against public participation). In those lawsuits, the goal is not to prove the story wrong, but to just waste news org's resources on defending their reporting in front of a judge instead of doing their job.