"What is not understandable is how so many liberals can blindly ignore 50 years of evidence to the contrary since then."
Maybe this quote from Carl Sagan will help us to understand:
“One of the saddest lessons of history is this: If we’ve been bamboozled long enough, we tend to reject any evidence of the bamboozle. We’re no longer interested in finding out the truth. The bamboozle has captured us. It’s simply too painful to acknowledge, even to ourselves, that we’ve been taken. Once you give a charlatan power over you, you almost never get it back.”