Because they're ignorant about what is and isn't the left/right dichotomy. And the small percentage that have even a slight idea (but still don't understand it) buy into the idea of The Great Swap, an idea that supposedly only applies to America, but they truly believe that all historical great evils of the left are now evils of the right for some arbitrary reason.

They don't care to look at the horrors of people like Stalin or Pol Pot. They're not interested in the reality that they'd agree with most of NatSoc talking points if it wasn't presented as NatSoc. They're not interested in the reality that Fascism is a threat to the left because it was explicitly an authoritarian reaction (and not one born from the left/right dichotomy) to the failures of current leftist doctrine, namely socialism.

They're not interested in the reality that the vast majority authoritarianism, paternalistic policy and general statism has roots in leftist doctrine. Because one day, they hope that they can be one of the people who can control someone else. Because deep down, that's what kind of person they ultimately are.