I think this is the evident reason. Anybody who's capable of any degree of self reflection can also probably see the exact problem in themselves. When I see a headline that confirms my own biases, I tend to be vastly less critical of it than one that challenges them. I have to consciously force myself to remain critical of things that say what I want to see, and that's very difficult - and something that the vast majority of people clearly do not do. And I think something that's really driving this problem is the deterioration of the media. When articles are increasingly regularly 'factually challenged' it let's people more easily kick in the cognitive dissonance and dismiss everything that opposes their own biases as fake.

And while you're giving some fairly radical examples, this applies to very simple and practical issues. Without stirring the hornet's nest with precise examples, it's safe to say that the reporting on most of any political issue is rarely even remotely impartial. So what is true and what is not? What we tend to believe, one way or the other, is what we want to believe - and you can certainly find some source or another stating it's true. Pair that with social media where people surround themselves with people who all mostly think the same way, and thus share a constant slew of sources all saying what these people collectively want to hear, and it leads sort of parallel realities where two large groups of people might see the color green, but one collectively convinces themselves it's blue, and the other that it's red.