By the fact that modern political and social theorists continue to refine and develop the theories behind this form of government. By the fact that the reason it frequently doesn't work is that capitalism can't allow it. A country where workers aren't exploited is essentially an untapped resource according to capitalism. Thus, the grey goo-like tendency of capitalism to take in all that is not it leads to an imperialist takeover of our little sanctuary of worker bliss.
Marx may very well have been right that we need a worldwide revolution in order to create this form of government in which workers receive the full value of their labor, and work is done according to what is socially necessary. Purge capitalism, and it all works nicely.
I've never seen a purely theoretical solution work the first time it's applied in practice, even in the simple, straightforward context of programming a computer.
I think the word "invented" has to mean something more than "proposed and refined a theory without any successful real-world applications".
Furthermore, Marxist theory has been tested. I think it managed to turn out worse than capitalism, which is quite an achievement.