The idea of voluntary communalism is an attractive ideal - when you see a piece of litter on the ground, you just do your part to pick it up and dispose of it, for example. In doing so, you are doing a public good, you are giving charity to the rest of us by taking on yourself the costs of picking up the litter yourself.
Totalitarian communism, by contrast, produces public goods by central control. The litter gets picked up because, ultimately, there is a faceless person in a remote Gulag somewhere who will execute you with a single shot to the back of the head and kick your lifeless corpse into a mass open grave if you disobey the government. And the government tells you when and where to pick up litter. Or make back scratchers.
I suspect that when leftists say "socialism has never been done right", they are expressing something akin to the belief that voluntary communalism is possible, it's just never been tried. It is logically possible for there to exist a society where people just do what needs to be done without regard to whether the individual doing the work is substantially benefitted by doing it, that is, a world where public goods are produced without the incentive of individual rewards. But logical possibility is a long ways from realizability.
Now for the puzzle. I find that most people believe that spontaneous, voluntary production of public goods is possible to one degree or another (I believe that charity is spontaneously produced in the absence of compulsion), but acquiesce to the "inevitable reality" that governments will abuse their populaces. I have been debating the issue of government coercion on discussion boards, and the refrain which I hear repeated again and again is that, if we want protection from foreign aggressors, we must accept some level of exploitation by those who are appointed to protect us. That is, while taxes and inflation may be indistinguishable from the identical crimes committed by private individuals (robbery and counterfeiting), we have to accept a certain level of robbery and counterfeiting in exchange for the security services which government provides.
What is puzzling to me about this is that someone who takes this position is implicitly accepting a principle of human behavior: that people will tend to coerce others to their own benefit to the extent they are able. In other words, governments will tax and inflate the currency to their own benefit because they can and we should not find this surprising. Yet, the belief that voluntary communalism is possible is founded on a rejection of the principle of human behavior that people tend to act in their own interests more than the interests of others. That is, voluntary communalism is possible because it is possible for humans - with the right education, culture or other factors - to shed their impulse to self-interest and simply cooperate for the common good. With armed coercion, we resign to the inevitability of criminal exploitation by the monopolists of power. But when it comes to economic competition, we feel that such competition is ultimately unnecessary because people can be expected to just spontaneously help one another, as a rule.
Violent coercion is inevitable but economic self-interest is not.
I suspect there could be an evolutionary explanation for this, however. People who refuse to acquiesce to conquest are more likely than others to die. So, there may be a selection pressure throughout the human history of war and conquest towards an inborn sense of resignation towards coercion, accepting its inevitability. Those born without this sense are more likely to perish without producing offspring. David Friedman supplies the other side of the equation in an article on evolutionary psychology and economics where he argues for an evolutionary origin for the popular notion of just prices. Basically, he argues that in economic competition for scarce resources, those who are more committed to "usual" prices in unusual circumstances (where demand or supply rapidly undergoes significant change), the party experiencing the shift is more likely to reap the benefits. If a buyer's demand increases greatly or a seller's supply decreases greatly then, by insisting on paying the "usual" price, the buyer reaps most of the benefit in the change of affairs. Vice-versa for sellers.
So, perhaps it is a consequence of human history that we naturally tolerate violent coercion while we bitterly resent economic competition, despite the obvious inconsistency of such a position. If anything, we should most bitterly resent and resist violent coercion while not only tolerating but encourarging economic competition since we each inevitably benefit from economic competition, while we each suffer from violent coercion (of anyone, even someone other than ourselves).