Its not that greed is human nature, its absolutely not. The left is correct about this. However due to human agency and variability in humans there will always be *that guy*.
You know the type. The one who ruins things by being a dick. The capitalists want to use that guy. This is also wrong as theres no way to make that guy happy. He will always want more, until he has it all.
The left needs ways to keep that guy from having power, and to keep that guy from gaining control of the systems that prevent him from having power.
Anarchism has to be a deliberately constructed system that prevents power accumulation.