While no system of governance is completely corruption proof, socialists believe that you can limit the scope of that corruption and the damage it's able to cause through democratic systems.
The problem with capitalism is that it's inherently undemocratic. Workers have little to no say in how things are produced, where things are produced or how goods/services should be distributed. This leads to the ruling class imposing rules within their own organizations that only benefit them to the detriment of everyone else. This, in turn, gives them the power to lobby governments for regulation favorable to them, subverting any democratic system we might have.
It's not just that this system is susceptible to greed and corruption; it literally rewards it.
Also, the idea that humans are naturally competitive is not supported by any factual data. If anything, when left to our own devices and without any external pressure, we're far more likely to form communities and cooperate with one another.
The idea that we're a competitive species comes from the fact that we've always lived under systems that forced us to compete due to the scarcity of resources (whether it's a natural scarcity or an artificial scarcity, as seen in capitalist systems). Given that humans are capable of producing far more than we ever have before, and the fact that we have more than enough to feed, shelter, treat and clothe every person on the planet, we're long past the point of this needing to be the case.
This is like a 360 Windmill Slam Dunk in the 4th quarter to ice the game
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