The reason you’ve never seen a ‘pure’ communist state is because communism doesn’t work. As soon as you try to implement it, you run into a number of problems that must be solved - usually at the point of a gun.
Communism doesn’t work because power and knowledge are very poorly aligned. It is impossible for a central authority to efficiently allocate the resources of the nation, because it doesn’t have the information required to know where the resources should go - nor can it attain the information, because it is locked up in the heads of the population and simply not available.
For example, let’s say you control the production of hammers. How are you going to distribute them? Just announce that the state is providing hammers, and let people come get them? In that case, everyone will want a hammer. I mean, hey free hammer, right?
So somehow you need some authority to decide who gets a hammer and who doesn’t. But that decision requires knowing who really needs one and who doesn’t, and that knowledge isn’t available to him either. Even if every citizen is being a good little Marxist and only asks for a hammer if he thinks he really needs one, you still can’t allocate them efficiently because the citizens have no way of knowing if their need is greater than their neighbor’s need.
Then there’s the problem of maintaining supply chains. If you dictate that hammer production must be increased, suddenly there’s a steel shortage, because you didn’t also allocate more steel production. Or maybe you were smart and did that too. But now the steel foundries are short of the goods they need to make the steel. So you order more of those goods, but the factories that make those are now short of the the things they need…
And so it goes. The Soviet Union had a planning division called “Gosplan”, which employed thousands of actuaries to try and manage the nation’s resources. They really, really tried. The end result was chronic gluts and shortages. Standing in lines was a way of life.
That’s just one reason why communism fails. Every time. To the extent that ‘communist’ countries survive, they do so by tolerating large black markets that re-distribute the goods that were poorly allocated in the first place, or they allow a certain amount of capitalism to co-exist, as China does.
Another reason communism fails is because of human nature. When you concentrate power in the hands of the state, you get corruption. Now the hammers don’t go to those most in need - they go to those who come from the right areas, who say the right things, who grease the right palms, etc.
And along the way, you discover that without some kind of incentive people just won’t work hard. Without reward they won’t take necessary risks. Stalin ‘solved’ this by hauling slackers off to the Gulag - fear is a pretty good motivator for a while, but it just teaches people to keep their heads down, and innovation stops.
The only other alternative was to pretend that everyone is equal, but to figure out ways to allow some to become rich if they worked harder or smarter. So the factory ‘owner’ might make the same ‘income’ you do, but he gets exclusive use of the state-owned limousine provided for the factory, and he is rewarded with ‘bonuses’ that include vacations to the black sea in the villa provided for use of the factory owner.
Or, everyone might earn the same money, but there’s nothing to spend it on in the common markets available to everyone. But if you’re a good little communist with the right connections, you get access to special department stores that actually have goods to sell.
To the extent that the Soviet Union worked, it worked because it went around the communist ideal and inserted enough of the features of the market to allow some semblance of reasonable allocation of resources and to provide some incentives for people to actually do anything.
Capitalism solves the problem of distribution by allowing people to negotiate with each other. A capitalist economy can be thought of as a massively-parallel supercomputer, with the price system acting as an information bus and an incentive mechanism. If the demand for hammers goes up, the price goes up. That forces everyone to re-adjust their own cost-benefit equations. People with marginal needs for hammers stop buying them. That allows the resource to be more efficiently allocated. Not just that, but as the demand for intermediate hammer products (wood for handles, steel for the head, the parts that make up the machines in a hammer factory), those prices then rise, which stimulates production of more of them. But that then drives up the prices of other products that use those raw materials, which adjusts the demand for those as well. Very soon, the system finds a new equilibrium and everyone in the chain of supply and demand is forced to re-calculate their needs in accordance to the new prices.
If you could see a real-time animation of a free economy’s capital flows, and you could change the color of the flow as the prices rise and fall, then with every change to the real-world supply or demand of products you would see the colors ripple and pulsate all through the economy as it recalculates and absorbs the new reality.
In the meantime, another aspect of capitalism that makes it succeed is the nature of innovation within it - millions of people incentivized to constantly seek improvements in processes, discover new resources, etc. At a macro level it looks like an efficient stochastic search algorithm. And when new things are tried, if they succeed capital flows to them, everything re-adjusts, and the new information is absorbed into the system. Things get more efficient. If new innovations fail, they fade back into the stochastic noise, and new things are tried. The ability to profit from your own innovation provides the incentive, and allows for the accumulation of capital which then allows for private investment in other innovations.
There is no way for any central authority to manage this feat, no matter how large or how smart it is, because it does not have the tools and never can.
In practice there are many other reasons communism fails, but those are the big ones.