humans are greedy and selfish

That's just what you choose to believe. These's no scientific proof that they make up some kind of transcendent truth about the human condition.

It's entirely possible that the humans traits of greed and selfishness tend to become much more pronounced when humans are subjected to systems that reward those qualities.

Also, greed and selfishness are distinct from self-interest. And besides, self-interest doesn't explain the entirety of human behaviour either.

Perhaps other systems that reward collaboration and egalitarianism and autonomy are not only possible, but also more sustainable that the shitshow we've got now, and all we need is for enough people to get out of the mental rut of believing capitalist bullshit about "humanity" and "life" just because it's the status quo.