Because many human beings are greedy and corrupt, people in communist countries just like in the rest of the world can and do when put in positions of authority abuse it and use the power they have to improve their own lot, to the detriment of others.
Then communism has the problem that we are not all the same, the redistribution of wealth would not remain equal, some people would spend all their money, others would invest it, use it to produce things and increase their wealth.
We are human beings, individuals, not ants who can all be set to work blindly for the good of our nest, community, country etc. to the exclusion of our personal wishes or wants.
When you take away human initiative then people cease to care, what is the point of working very hard when you can achieve no more for yourself or your family than the person next door who does as little work as possible.
My father was a lifelong communist and if everyone in the world were like him then communism would work. He tirelessly stood up for equality, civil liberties, workers rights etc. he had a central belief in justice for all. This involved writing to Chinese and Soviet organisations to complain about their human rights abuses. However the world is made up of many who do not care about these things. those who don't give a damn about their own countrymen, let alone those in the rest of the world are common in every country in the world regardless of their political system.
This is something I spent many, many enjoyable hours in debate about with my dad, he had this idea that all human beings were essentially good and cared as he did about every individual and that if communism in it's true form was ever established, it would thrive. I thought then as I do now that humans have a lot of evolving to do before this can happen.