Wikipedia notes: The initial usage of the term capitalism in its modern sense has been attributed to Louis Blanc [a socialist] in 1850 and Pierre-Joseph Proudhon [an anarchist] in 1861. Karl Marx and Friedrich Engels referred to the capitalistic system (kapitalistisches System) and to the capitalist mode of production (kapitalistische Produktionsform) in Das Kapital (1867). The use of the word “capitalism” in reference to an economic system appears twice in Volume I of Das Kapital, p. 124 (German edition), and in Theories of Surplus Value, tome II, p. 493 (German edition). Marx did not extensively use the form capitalism, but instead those of capitalist and capitalist mode of production, which appear more than 2600 times in the trilogy Das Kapital.
In short, socialists invented capitalism, which is to say, they invented an ideology as a foil for their ideology. Once we step outside the ideology of socialism we do not find the ideology capitalism but instead economic liberty and attempts to deny economic liberty. Socialism is an ideology which (among other things) attempts to deny economic liberty and in its place implement an ideology called socialism.
Ideologies are inherently idealist in the philosophical sense of asserting that reality is mental or immaterial. Most ideologues are idealists as the words would imply, but some – notably Karl Marx – claimed to be materialists. Either way, ideologies are inherently anti-realist.
Realists (at least as realists) do not promote ideologies but instead independent realities that are discovered, not invented. Economic liberty was gradually discovered and matured into modern markets and finance. Over time it is inevitable that some people will accumulate more wealth than others as long as economic liberty allows people to express their talents and inclinations. That can cause social problems which may legitimate state intervention.
That is not an endorsement of the ideology socialism but a recognition of the complementarity of liberty and equality in society. A realist response on how to reconcile these two priorities would follow a dialectic of complementarity to find a satisfactory mean between the extremes. Instead what many societies are dealing with is a dialectic of contradiction which tries for an extreme of liberty or (more often) equality alone. This is a prescription for instability, unsustainability, and worse.