Because that’s how concepts come about, especially ones that are teleological in nature. Capitalism is like that to. Historically, as most people refer to it, capitalism refers to the type of activity going on in the industrial revolution. A big part of that is trade and wealth creation. Labor is part of that as well, since goods have to be made. Trading of capital is important, too. The purpose generally was about creating more capital. I don’t think even Marx referred to capitalism as anything different. Then you look at what happened as a result of the industrial revolution, seeing what aspects are due to capitalism, or other factors besides capitalism. I’m not going to induce everything I know right now, but eventually we get to the point where it’s clear that capital helps lead to wealth, and thus self-interest. You would need the history of rights, too, to establish what rights are good for, and why someone came up with the concept.
Have you ever had a “conversation” with a socialist about capitalism? If so, I imagine it proceeded much like this conversation with Devil’s Advocate re: marriage. You’ll hear lots of “history” about the awful things that supposed capitalists have done, and how governments have intervened (and continue to intervene) on behalf of industrialists, and etc.
You’ll say something about how capitalism is the economic system that exists in a state where individual rights are protected, and how none of that is actually capitalism properly understood, or so forth, and your socialist partner will say, “hey, I’m just talking about the history of capitalism, which you must understand to know the true nature of capitalism.”
What stone age folks did for marriage, or Edwardian England, or etc., does not matter to what marriage is currently, or what it ought to be. You don’t have to go back too far in US history to find anti-miscegenation laws, and such. It was all very traditional.
But tradition is a poor excuse to act in this manner, because it is discriminatory and does not speak to the role or function of marriage in modern society. The reasons I had to marry my wife, any gay man or woman might have for their partner. And thus they should have access to the same institution. If you think that “history” somehow demands otherwise, then “we the living” must simply disagree with you.