People see what they want to.
Some persons look at capitalism and they see self-interest. Some even call it rational self-interest. Three big words. Rational. Self-. Interest. Is there any part for sociality, or interaction? Why are those ("social" -ist) things supposedly separate from capitalism? Those things may appear to you to be separate things from capitalism, that is to say if you are indoctrinated.
YET ---- EVEN SO ---- Everywhere we look, humans are found in a society. The truth is this. Self-interest is not really even that interesting. They may like to put self-interest and capitalism together. That does not make it true. To put the two ideas or concepts, capitalism and self-interest, together rhetorically is to do something intentionally. It is something someone did. Rhetorically, even. But, to put to things together does not prove a relationship.
We are just assuming it. The orthodoxy in the universities are so stupid, it is just awful. It's horrible. We are just assuming this relationship (capitalism and privateness or self-interestedness) is there. Their thesis is, more precisely, that capitalism is always associated with self-interest. Maybe that looks good on the surface, to the adherents of this view, just because we do live in, after all, a neurotic society. In this repressive society, all kinds of selfish idiots are running around in the streets preening and driving Lexuses and showing their feathers --- or, their latest dress shoes and cuff links. Wouldn't it be nice if there WAS an entire human system based on self-interest? Then they'd all be doing the right thing.
Yet ---- and even so ---- people do live in societies. Now, how is it so certain that in fact sociality -- the aspect of human socialness -- and capitalism are actually all that separate.
Maybe they are together. Maybe very intimately involved with one another. aAnd maybe that scares selfish people. Maybe whenever capitalism is successful, the persons in that society feel a social bond. And the selfish and greedy do not like that.