The Communist Manifesto and Das Kapital is almost devoid of any mention of immorality and violence and human impulses. There is little understanding of how social relationships and systems actually form, it is like trying to learn about chemistry without any mention of molecular structure and straying off into alchemy and the 4 elements.

Imagine a merchant who acts as a middleman between a spinner and a weaver handling the numerous transactions, distribution and storage, all of which are an economic expense alongside the actual spinning and weaving. The communist however would say something like the capitalist superstructure forces the spinner to sell to the merchant who sells it at a higher price to the weaver and pockets the difference for zero real work. It is possible the merchants are all in cahoots and formed a cartel to manipulate prices of course, and one might imagine the spinners and weavers forming a union to counter this then attempting to centrally plan the distribution of spun wool and other matters like how much to spin, the type of wool, how it is spun and who does the spinning. However you then run into the problem of incompetence, fraud and scams within this union, favoring spinners over weavers or one group of spinners over another, granting the leadership privileges above everyone else and so on..

This is perhaps the origin of this error. These unions want to be viewed as perfect and infallible. Despite their grass roots origins looking out for the "proletariat", they fall prey to the exact same self-justification as every other large political body. So naturally when Karl Marx set out to write his treatise he does not mention this, perhaps deliberately. If you want to start a revolution, you are better off reading Hobbes.