It’s because capitalism is the natural state of man - that is, something that he produces goes to benefit him, while at the same time benefitting the community around him, and he defends his territory or property and uses it in the production of his goods. As social organization moved away from its tribal form and, with the advent of currency and a money based system, became more individually controllable and also more flexible (making capital more easily transferable and resulting in the rise of a moneyed class rather than merely an hereditary property owning class), it did so naturally and with little pre-planning. Property rights are based on the fact that ownership is natural to man. But because it’s a natural and therefore somewhat amorphous, unstructured thing, it’s harder to enunciate.
Socialism, on the other hand, is an artificial system that must be developed by an originator (Marx, for example) and imposed by force, since it is unnatural. However, precisely because it’s artificial, it’s easier to enunciate and, while it has never worked in practice, it’s easier to defend because one can always claim that it’s not being implemented properly.
The only people who benefit from socialism are those in charge of enforcing it, that is, those in government. This is something pro-capitalist thinkers need to point out unceasingly. Socialism is repression and never benefits the poor, even when it’s “soft” socialism such as in England, where socialism has created a huge, festering, going -nowhere class of the native-born poor, who have enough to live on and miserable government health care, but absolutely no prospects. Still, the government and everybody who is associated with it thrives. And in a more harsh socialist system, everyone who is not in the government or favored by it is absolutely crushed into the ground and deprived of freedom and even life. That’s a pretty good argument against it.
Right, capitalism came about spontaneously through the natural flow of things whereas socialism was posited by intellectuals. Because socialism is a creation, all of its details can be known by its creators. On the other hand, we may never fully understand all of the organic subtleties of capitalism. The difference in socialsm and capitalism is sort of like the difference in a cartoon character and an actual person.