Its because they genuinely believe their old "reality has a liberal bias" phrase.

Its not a slant for the entire rest of the media to be in lockstep, they are just reporting "reality." Fox is the odd man out because they are reporting "distortions of reality or lies."

Like, that's the heart of all their worldviews. They truly believe their vision of the world is the unfiltered truth, and anything that isn't congruent with that is someone trying to trick them.