The answer is because they shield themselves from reality with Olympics level mental gymnastics, rationalizing away cognitive dissonance and living in their own deluded reality.
Oh my, no. It's nothing that complicated. These are conservative minds, and the entirety of their worldview can be boiled down to one question:
Is this good for us?
It has nothing to do with denial or cognitive dissonance, because they simply are not thinking that hard about anything.
Is this good for us?
If the answer is yes, then everything else is negotiable. Reality is negotiable. Nothing is absolute unless it absolutely benefits the conservative self.
Is Trump a liar? Is Trump a serial rapist? Is Trump a national security threat?
Is it good for us if he is? No, so he isn't.
Or maybe he is, but it's not that big of a deal because it is better for us if it's not a big deal.
Was the election stolen? It would be good for us if it was, so it was. We don't have to prove it, because we already know it was because it would be good for us if it was. Why are we still arguing about this? Arguing about it is bad for us, so just accept that it was stolen and anything anyone did trying to prove it was fully justified because it would have been good for us if they had been successful.
Is abortion murder? Calling it that gets angry people to vote for us, so it is. Medical science? That's not good for us, so it's not real. Maybe it's a conspiracy or the devil or immigrant trans lesbian atheists, but that's not important right now so there's no need to defend any of that ridiculousness.
You cannot argue with a conservative. You cannot point to the flaws in their logic, or provide concrete evidence that contradicts their beliefs. Changing the mind of a conservative is like changing the direction on a revolving door. You're still either in or you're out.
Realize this, and accept that you are not dealing with a rational ideology. It's a dog with a ball that wants you to throw but won't let go of the ball. It's not complicated, there's no deeper hidden communication coordinating all of the conservatives to orchestrate a big conspiracy. It's simple narcissism. And it should be treated the same way you would treat a narcissist.
Set boundaries. Ignore theirs because they are not fixed.
Justify your boundaries with facts. Ignore their arguments because they are not based on facts. You're not trying to convince them, you are justifying your position for the next rational person coming along. You don't need to disprove every one of their firehose of bullshit arguments, you just need to prove your case and stand your ground.
Do not compromise or negotiate with a conservative. They will not be satisfied with whatever you give up, and they will take whatever extreme position is necessary to make the "middle" wherever they want to be.
Demand your rights, and accept no substitutes. The law exists to protect each of us from tyranny, and the conservative craves the position of tyrant. Your rights will not be given to you by a conservative, you will have to fight for them.
Remember this, and you will not be surprised by the hypocrisy of a conservative. You will never be confused about how they could possibly ignore facts and believe what they want. Your expectations will align with reality.