I believe our societal models are the reason. It's not just that all people are self-absorbed and greedy, it's that everything we have built for ourselves pushes us toward being just that, from start to finish.
We have slowly snubbed out true values, like a sense of justice (by this I mean fairness mostly, true Justice, not Electric Chair Jamboree), empathy, collaboration, soulfulness, all of the noble qualities of humanity, and have slowly replaced them with therapy shopping and a fixation with achievement in order to cover up that which is missing from within ourselves.
Then again, some are just... empty. Devoid of personal meaning, which is why they want to empty everyone around them in order to not be alone with that hole in their souls.
We have created a parallel reality. A consensual dream. And, when you live in a society, skill at navigating the consensual dream is more useful than skill at navigating reality.