Because humans experiment with societal rules as societies were developing and get into self reinforcing loops that go on long after everyone's forgotten why it happened in the first place.

Human nature is to form societies. What happens in those societies and how they are structured are the result of chaotic interactions and competing thought that, again, are the result of material conditions those humans find themselves in.

There are plenty of societies that don't strictly follow the Roman/European system of power. Japan for example had their emperor reduced to a symbolic position long before European contact, but even though the emperor had most of his real power taken away, everyone still called him emperor and worshipped him because he was so important to their culture, power or not. Meanwhile, in what would be modern day India, multiple different religions arose based on selfless sacrifice for others and rejection of indulgence and pleasure in favor of self reflection and simple living, with many people throughout history in the region (princes, heirs of family fortunes, etc) fully rejecting their very privileged lifestyles to embrace aestheticism. Same with ancient Greek stoic and cynic philosophers many of which came from rich and powerful families yet deliberately choose to reject all of it. That all seems pretty against "human nature" no? Then you had the Indigenous tribes of the world who practiced small egalitarian societal groups and did perfectly fine until Europeans intervened.

You can argue that human cognition allows for the world we see around us, but that doesn't mean this was necessarily unavoidable, or that we're naturally selfish, or whatever else. It's also very self-defeating as an argument because we can point to countless examples of cooperation, both past and present, including people saving someone from drowning for no monetary gain. And I don't know you personally, but I try my best to be a good person out of love for humanity, rather than expecting a reward.

But let's say that it is human nature to be selfish. It's still a silly argument against socialism though, since we could still build systems that don't reward our worst Impulses, but rather function with checks and balances to prevent them.