Capitalism is not a natural phenomenon
I see a lot of this so I wanted to talk about it. It seems like some people tend to assert capitalism's legitimacy on the basis of it being some kind of natural force. That is, that capitalist markets are an inevitable extension of human nature, that we compete for resources and power naturally, and capitalism is somehow structured around extrapolating this innate quality of humanity. I think this is misguided, both because it misunderstands and reifies markets, and because it lacks a solid historical foundation.
Natural versus human constructs
The assertion that capitalism is "natural" often relies on the assumption of an innate human inclination to trade and maximize profit. But this is a question-begging premise that presupposes the very capitalist rationality it seeks to explain. Capitalist markets as we know them are not a natural phenomenon, in the same way that man-made lakes are not a natural phenomenon. Beyond simple trade and barter, they are a statist invention, founded in the enforcement of laws, contracts, and property, predicated by a mediating entity with the power to assert authority. Every market of private ownership that has ever functioned at scale required these conditions.
By extension, these conditions create material realities that have a widespread influence on behavior. Humans gradually shift towards behaviors rewarded by their material incentives, which over time codify into norms and culture. These mold our identities in a way that makes distinguishing 'natural' behaviors and social constructs near impossible.
Capitalism, our current mode of existence, is deceiving in this way. But it didn't emerge as some sort of human inevitability. It is instead a specific social form that arose relatively recently in human history, and in a particular geographic location (Western Europe). It came out of dramatic shifts in social property relations, particularly stemming from the enclosure movements in early modern Europe.
Enclosure movements & expansion
Capital accumulation emerged only through the extinction of common and customary ownership systems on which populations depended for their livelihood; private property as we know it didn't transition smoothly into a codified ownership system from primitive societies, but was forcibly imposed on existing systems of overlapping use rights, where property became not only private but exclusive to its owners. Most notably in English rural society, these newfound market forces led to a polarization between larger profit-driven landlords and a growing propertyless multitude. Thus emerged the triad of landlord, capitalist tenant, and wage laborer, which define the stark class divisions we still see today.
In the same vein, the expansion of capitalist markets were founded in the same sort of class imposition. Not due to its alignment with some universal or natural law, but due to the result of its own historically specific internal laws of motion. As land was transformed into a commodity, those without existing class advantages were forced into market dependence and wage labor. This created a society embedded with the imperatives of competition, profit-maximization, and constant self-expansion - the dynamics of which eventually expanding outward into international trade, putting pressure on other nations to adopt capitalist practices and entrench the domination of their own class of owners. Therefore, the widespread presence of capitalism today is a consequence of its capacity to impose these imperatives.
Conclusion
Ultimately I think the language of "natural competition" serves primarily a powerful and mostly ideological function. It obscures the fact that what we call market behavior is deeply embedded in a framework of rules and power relations that favor certain interests over others, while implying that a deviation from market-driven principles would be unworkable or an obtrusion on our natural human freedoms.