Another part that adds to this notion is that libs generally only encounter crudeness from literal children or right wingers.
They think the real world politics is some West Wing-ian paradise of polite discussions and good faith arguments. Only MAGAs resort to ad hominem, ofc, Donald Trump is bad partly because of bad policies but mainly because he's crude! Good Republicans are polite and also want what's good for the country, they just disagree on methods!
This probably stems from anti-civil rights movements saying the blacks are impolite or crude when demanding basic human decency.
Also historically untrue, on at least one occasion, Abe Lincoln bullied an opponent to tears with ad hominem attacks.
Either way, they fully buy into the scam that politeness and taking the high road will win the day except the part where liberalism has been in decay for decades due to what basically amounts to stomping your feet at conservatives every time they slide into fascism.
And they keep doing it, Sisyphus with the rock, Charlie Brown with the ball, libs with politeness. Guess what? Not disrupting evil in any meaningful way means they get to continue to make things worse and get away with it.