Leftists typically try to understand conservatives in a way that is not true, for the purpose of disagreeing with them. They can’t openly disagree that limited government is better than big government, so they call conservatives racist and Nazis instead. In doing so they completely underestimate them. One of the biggest mistakes in war is to underestimate your enemy.
“Leftists typically try to understand conservatives in a way that is not true, for the purpose of disagreeing with them.”
That’s part of it, but that’s not all of it.
Another aspect is that leftists are regularly dishonest about their own political beliefs. They think the ends justify the means, so they believe if their goals are lofty, then they can adopt deception as a justifiable tactic, and market their politics deceptively. Then they assume that conservatives are also being deceptive, so they balk at taking our stated goals, principles and arguments at face value for that reason as well.