Because the news media has zero credibility, they don’t really have anything to lose by doing this. It really is an outrageous crime against what was at one time a fairly honorable profession of western journalism.
NBC isn’t selling their credibility and clearly doesn’t care about it.
Their business model is to attract eyeballs to the ads by making fun of conservatives.
Since their audience is barely measurable in many time slots it doesn’t seem like a very good business model but so be it.
But they can not prove absence of malice.