humans are not naturally greedy, but clearly there are some greedy or power hungry humans, which can be seen through the existence of billionaires (and as most have likely met someone they consider greedy), and just a few of these people would be enough to prevent a system from communism from working. this is, in my opinion, the reason that every past large scale attempt at communism has eventually turned into some form of authoritarianism, and the "provisional" government almost never is actually provisional, as the people in it don't want to let go of their power. it is hypothetically good, but it has been shown multiple times not to work, and just attempting it again and again isn't going to help