They're just politically and historically uneducated which results in an incoherent worldview. I passed through a phase like this myself when I was in the process of rejecting liberalism but my head was still swimming with red scare nonsense, which left me politically confused, wanting an idealistic equal and empathetic society but not yet understanding that these "authoritarian" aspects would (and are and were, historically) necessary to defend it. Part of that was because I had not yet understood the full extent of the barbarity of liberalism, even though I understood it in part. Parenti has a famous quote from one of his lectures that went something to the effect of, "If there was no NATO, there would've been no need for a Warsaw Pact. If there was no CIA, there would've been no need for a KGB." I think people in this stage of moving left (whether or not they eventually become communists) don't have a grasp on the enormity of the attacks, subversion, and siege levied against socialist states that can only be defended against with structures that are unfortunately capable of grave abuses. In the same lecture, Parenti uses the example of the Nicaraguan revolution, saying something like (not an exact quote), "They discovered a very terrible thing. They discovered they needed a secret police. They discovered they needed Instruments of coercion, and these instruments are capable of very serious crimes." Finally, though I don't necessarily see it in OOP, there can often be an element of racism as well. The Soviets and PRC will be seen as "authoritarian" while Cuba will be seen as okay, despite Cuba (until recently? I haven't kept up with the reforms over the past couple of decades) being a Marxist-Leninist state more or less identical to Soviet governance. This is exactly the goal of the red scare(s). It's not to outright prevent everyone from moving leftward, they'll never achieve that, but those that do so have been deliberately maleducated so that they do not align with the actually existing (and therefore working, and therefore threatening) socialist projects.

tl;dr, political and historical maleducation makes people say some stupid shit.