As others have pointed out, a lot of this is just mindless parroting for the sake of optics, but I can confirm from my personal life that a lot of otherwise rational people are actually denying reality. A difference from normal conspiracy theories I've noticed is that these aren't held deeply as deeply held as mainstream ones. The people I've talked to IRL about this haven't put up a lot of resistance to basic counterarguments like you'd expect from an actual Truther. Partially, I suspect this is because there isn't a wealth of detail that fuels continual scrutiny and most of the theories originate from arguing on Twitter over blurry cell phone footage.
My personal theory is that this whole phenomenon is the end result of severe cognitive dissonance on a mass scale. I'm not sure if there's any historical precedent for the kind of psychological reaction we're seeing. Liberals are simply unable to accept that this is what they've been reduced to: cheering for suicidal nutjobs to commit politicized gun violence. It goes against everything that has defined their political views for the last 30 years. A lot of this is because of their entrenched belief that we have managed to successfully to put ourselves on track for the Star Trek future exclusively via peaceful social change. Some influential examples of this line of thought include The End of History and the Last Man, and The Emerging Democratic Majority.
