That goes beyond capitalism. People are just selfish. The hoarding of wealth was a thing way before capitalism. I think the left sort of shoots itself in the foot by obsessing over capitalism and ignoring the much deeper cause of a lot of societal ills. Being evil is part of human nature, just as much as being benevolent is.
There have been extensive sociological studies over this. Condition in a capitalist society and the promotion of the “homo economicus” model continually reinforces “greediness” and leads to people in capitalist societies being far “greedier” on average.
It isn’t a natural thing, it is conditioned. Obviously everyone is greedy to an extent. But in anthropological examinations of different forms of societies, altruism scored far higher than greediness in non-capitalistic societies.
Kate Raworth, Oxford Economist, wrote an excellent chapter about this in her book called “doughnut economics”. The chapter is “Nurture Human Nature”.
The view that all humans are greedy and rational was promoted by Adam Smith and John Stuart Mill and is the precursing foundation of capitalism. But modern economics have rejected this view as it has been proven to be inaccurate, and increasingly rely on theoretical models built within behavioural economics.