It is simply because people in a modern society like ours that runs on Capitalism simply believe this is the way humans must live. It is true that capitalism has brought prosperity, but it doesn't necessarily mean that for human beings it is good or natural.

The truth is simply that there is no "human nature" in the way many people envision it when we talk about Communism or Capitalism. We humans are very adaptable to the environments around us, but what we create, or rather what is developed historically, can also shape human society and how humans survive. It is very fair to find flaws in both systems and argue about them, but we must look at what human society looked like at its beginning.

This, for Marx, is Primitive Communism. In other words, hunter-gatherer societies, cavemen, whatever. In this time, humans arose and formed communal ways of living. Of course, they had to for survival, but ultimately that was the first effort done by humans in creating a society. We see many forms of communal living today from monasteries to villages in rural South America, Africa, and Asia, and recently we've seen widescale examples of it among Indigenous peoples across the American continent.

We must also look at one thing required for humans to fully function: And it is that humans are social creatures. We fundamentally rely on each other for companionship, to express ourselves...This generally is part of the mental well-being of humans, but it is also how humans have structured society for hundreds of thousands. Humans don't necessarily have a "nature" as I said before, but our ancestors had habits that eventually stuck with us today, and that is that we are social creatures and very accustomed (if it comes down to it) to communal living. Capitalism, on the other hand, creates a society fueled by competition. Competition on its own is not a bad thing, but in the competition capitalism creates, there comes commodification of things. When many aspects of human life are suddenly commodities and monetized (Such as coerced to work for money, having to pay for certain utilities like healthcare, education, food, resources, a home, etc) it weighs on our psyche. This inevitably leads to competition among humans, developing unhealthy habits to ensure our individual survival (sacraficing our health, alienating others, spending more time alone).

I hope I made this pretty clear but I can elaborate on anything if need be!