It just doesn’t scale up at all.

Exactly. In the context of a small tribe, a family structure, a friend group, or a small commune, communism works. Why? Because there are social methods of enforcement. That is, if you're a greedy dick, everyone else will know and ostracize you for it. Thus, you have an incentive to play along fairly.

But once you get to a larger society — past Dunbar's number — you can no longer keep track of everyone and whether they're trustworthy or not. This allows bad actors to not play fairly with minimal consequence, breaking the system of relationships and trust that had allowed the system to work in the first place.