It's a tool, not a reason. If there were no fiat money, there would be other tools. Greed is the reason. If you somehow removed all currently rich people, in 20-30 years you would have another batch of super rich.
Greed is just a proxy word for human nature. Humans are the reason.
Now you might read this and think "This isn't true, I'm not greedy". I'm guessing 99.99% of people reading this thread have more money and food than the average human on earth. Statistically if you reading this you are greedy relative to the average human.
