[ 8/6/20 ]
Some aspects of the answer given here by others.
It seems clear to me that the answer is deeply complex and contains some genetic elements, some cultural elements, some logical elements and some strategic elements.
Part of it is that we have very long childhoods, in which we are not capable of surviving without help. Thus we must have strong social bonds for the species to survive.
Another aspect is that our species is defined by its use of technology. We trace human development through the fossil and archaeological record by the fragments of the tools and technological products that we leave behind. Most technologies work better in collaboration. One person can learn to do many things, but if you have to start a forge every time you want to repair something metal, then it will take much more time and effort and resources than if you have a forge running continuously and are doing many different jobs for many different people.
Another aspect is that there are many different classes of threat that can be most effectively combated through cooperative effort. And that comes with the twist that naive cooperation is always vulnerable to exploitation, and thus requires sets of attendant strategies to detect and remove and “cheating” strategies on the cooperative. And that can evolve in complex strategic ecosystems at every level.
Another aspect is that all new levels of complexity require a new level of cooperation to emerge, and because of the need to counter cheating strategies, that gets complex. We human beings seem to be the result of at least 15 levels of such complex adaptive cooperative systems.
The complexity of our language, culture and technology cannot be created by any single individual, and require fundamentally cooperative contexts to sustain them. Viewed from a systems perspective all moral systems can be seen as some approximation to a required set of boundary conditions for the survival of complex cultures. We are so technologically competent that all out competition is self terminating and must be avoided (a great danger from competitive market systems in the presence of automated technologies).
Another aspect is that freedom always finds its greatest expression in cooperative contexts. Looked at systemically, competitive contexts always tend to force systems to the lowest available space on the complexity landscape. This is the exact opposite of freedom. Cooperative systems allow the exploration of new “spaces” of complexity (which comes with both risks and benefits).
It is deeply more complex than this simple overview suggests, and it does point in some strategic directions that those interested in systems and strategy can follow if that is their interest.