This factor is so critical we've named an entire field of science dedicated to this
And the thing is, we also know what success is in Monopoly, and subsequently where those paths lead us: complete monopolistic domination over your opponents.
Everyone playing Monopoly who wins does so by being the most selfish, greedy, uncooperative, most competitive individual they can be.
Most people who play (and win) at Monopoly do not act in those ways when they are not playing Monopoly.
If your friend whom you deem selfless and friendly beats you in Monopoly by being ruthlessly greedy, does that mean they are ruthlessly greedy?
Ruthless greed is not just incentivised from its players, it is mandated.
Your closest loved ones and friends will still act in ruthlessly greedy ways when playing a game of Monopoly.
Well, it's because the belief that humans are greedy is substantiated with evidence of humans acting in ruthlessly greedy ways...in systems that reward ruthless greed.
People don't act for no reason.
Well, it was that the West forced their ports open, and suddenly Japan found itself playing a new (Western) game, with rules and norms that pushed for certain behaviours and defined a Western definition of success.
China didn't do what Japan did at the time. The system ultimately punished them for it.
You cannot come to the conclusion that humans are selfish, without first ruling out that they are only acting in that manner because the system they participate in mandates such behaviour.