I always laugh when I see someone bring up Fox News in this context. You realize your just proving his point, right? I can name dozens or hundreds of major media outlets that that lean left (MSNBC, CNN, NYT, WaPo, HuffPo, BuzzFeed, etc.), but you can only name one or two that lean right.

Most of the "left-leaning" organizations you cite above are not considered particularly left-leaning by the left. A better description of them would be "pro status quo", or if you're feeling punchy "relentlessly pro status quo" with a side of "pro-corporatist" thrown in for seasoning.

Now, Fox News and OAN and Sinclair are not, publically, actually the John Birch Society, so sure, you could say "right wingers don't think they lean right either".

Plot out a chart of US political opinion, and I more or less guarantee you that in most things, you'll find your list of media outlights absolutely "centrist", and the 3 I just named distinctly "to the right" of mainstream US political opinion.

Fox is the most familiar example, but there are many more:

https://www.allsides.com/media-bias/media-bias-ratings

If you're talking about people like Chris Wallace, then I agree with your point. People like him are very much an example of The Cathedral but on the conservative side. Even stuff like The Five is pretty middle brow and establishmentarian but they aren't the vanguards of mainstream conservatism like Maddow and Joy Reid and others are for mainstream liberalism.

Steve Bannon and his movement are more representative of mainstream conservatism. Even someone Tucker Carlson used to be very much like Chris Wallace. The reason why his program is so popular now is because he's shifted radically away from his establishmentarian position towards Steve Bannon's position.

Personally I don't watch Fox News because of how low to middle brow and Cathedral-like it is.

If Maddow is the vanguard of mainstream liberalism, why are her ratings so terrible?

You can only get a plurality in most cases with mainstream media, because like it or not it does tend to be progressive/liberal in most outlets (in the American sense, not European).

That doesn't explain why FOX is twice as watched as MSNBC.

NPR's drivetime radio shows have bigger audiences than Fox, for almost all of its shows (and sometimes, for all of its shows).

Cable TV is still a niche.

Probably because the Cathedral of mainstream liberalism has been mortally wounded over the past decade. That has rapidly accelerated as of late and you're seeing the last dying breaths with the rise of populism on the left and right. Maddow used to have much higher ratings. AOC for example is not part of mainstream liberalism. She is at the vanguard of mainstream popular leftism.