They act like the ideology is static and everyone agrees with each other and how things were in the past.
They act as if liberals and conservatives weren't lynching minorities a few decades ago, then spend millions of dollars trying to court them to their side today. They act as if they weren't sterilizing you for being disabled or gay for centuries, then spend millions encouraging corporations to throw parades for them. American liberals act as if their best selling broadway show isn't about pro-American POC playing as historical figures who would've put a chain around all of their necks and sent to the fields.
I don't mention these things to defend socialist countries that had the same or similar policies towards minorities. But it's fucking annoying to see that the sentiment that only capitalists are allowed to evolve and change their stances on things. "Stop crying about slavery, that was hundreds of years ago. But every sin under communism must be scrutinized because everyone today still supports the same exact beliefs a hundred years ago."
I think that there are many probable reasons. Partly, I think they need a way of feeling virtuous about benefiting from and supporting a system that is obviously monstrous (i.e. liberal capitalism). The nagging question, 'why are you not a radical?' becomes far easier to answer if you can say, 'despite the appearance that capitalist exploitation ruins countless people's lives and perpetuates the worst hatreds and divisions in society, getting rid of that exploitation would actually lead to something worse'.
But I'd also argue that a big one is probably just the effect of ideology - the fact that the system that people live in conditions their experience and sense of reality. People's experience of the world within capitalist societies is of exploitation and discrimination as the basis for social organisation, I think this just inevitably leads them to project these forms as an essential truth of human societies and makes it difficult for them to imagine that communism could exist.
Ultimately though, I think /u/AHopeOnceMore is correct to say that it's largely the result of 100 years plus of anti-communist propaganda making anti-communism a reflex response for liberals