It's way simpler than this. They simply cannot admit that Lenin, Stalin and Mao just got communism wrong. They refuse to move on from those authoritarian traditions. That's really it.

Marxism, in general is a very modernist political theory. For most of the 20th century, we extended most modernist ideals beyond that rigid structuralism. Leftism is no exception, but MLMs in general refuse to move past that way of thinking. As you say, it is very similar to the way that modern conservatives seem to hold that early 1900s thinking up as some philosophical ideal.