I think part of the problem is they simply don't like the news that's being delivered, and don't realize that journalists have a professional duty to report accurately on current events.
They also take the wrong lesson from the rare occasions where it turned out that didn't happen, and the journalist was fired and blackballed as a result of lying or distorting their story to attract notoriety or advance an agenda. They think it shows a pattern, but it's actually an exception that proves the rule.
They like to harp on things like the fact Ronan Farrow was fired by NBC while he was investigating the Weinstein scandal, while ignoring the fact that he simply took his story to the New Yorker instead and that magazine and the New York Times later published his story and received Pulitzer Prizes for their work. This story was a triumph of the free market and of the mainstream "liberal" media holding a bad actor accountable. https://www.businessinsider.com/ronan-farrow-nbc-harvey-weinstein-investigation-timeline-2019-10
They also conflate the distinction between fact journalism and opinion journalism.
Mainstream media isn't infallible. We all realize that. But responsible citizens have to stay informed somehow and it's still better than the fringe alternatives.
Random bloggers, YouTubers, Russian trolls, and the like aren't accountable to anyone and don't have to suffer any consequences for the nonsense they spew.
