Try doing anything without other people being involved, is that even possible? How does that line of thinking tell us anything about how people should relate to each other? You can analyze atomized individuals, but you end up with really incoherent ideas like "humans are fundamentally selfish". Sure, humans can be raised to be selfish, but compassion has to be beaten out of people in order to maintain a system that relies on these ideas. Honestly, the problem with individualism is that it depends on people living in an extremely forgiving environment. Any amount of serious adversity will teach you that it fucking sucks to do hard or complicated shit by yourself.