This is all about Dunbar's Number; the number of people you can "know" in the sense of having stable social relations with. Its about 150 (https://en.wikipedia.org/wiki/Dunbar%27s_number).
A society smaller than this can be effectively self-policing because everyone keeps track of how everyone else behaves. No money, police, laws or prisons are required because everyone knows what everyone else thinks of their behaviour and wants to stay on the good side of the community that they depend on. So its not a problem.
Once a society grows past Dunbar's number this system breaks down. You see people you don't know misbehaving, but they don't suffer any downside because nobody knows who they are. When you have to deal with people you don't know if you can trust them, but you do know that if they rip you off you won't have any comeback. So the communal trust breaks down and you start needing police, courts, laws and prisons.
In short, anarchism and "primitive socialism" are nice, but they don't scale.
A society smaller than this can be effectively self-policing because everyone keeps track of how everyone else behaves. No money, police, laws or prisons are required because everyone knows what everyone else thinks of their behaviour and wants to stay on the good side of the community that they depend on. So its not a problem.
Once a society grows past Dunbar's number this system breaks down. You see people you don't know misbehaving, but they don't suffer any downside because nobody knows who they are. When you have to deal with people you don't know if you can trust them, but you do know that if they rip you off you won't have any comeback. So the communal trust breaks down and you start needing police, courts, laws and prisons.
In short, anarchism and "primitive socialism" are nice, but they don't scale.