It’s worth noting that the grand majority of Americans are capitalists. Most of us think that it’s perfectly reasonable to be paid in accordance with our labor, to be able to have private property and possessions, and to allow the market to determine the optimal way to create and distribute the things that society needs. So overall, I’m a bit curious about the way the OP is written. It seems to be implying that there’s only a small percentage of the population practicing capitalism.
But yes, capitalist thought was first codified by Adam Smith, a philosopher. He noticed that nations could war for territory and view everything on it as a limited resource which, the more you had, the better off you were; or nations could trust to technology and specialization to trade with each other, preferring to better the world rather than trying to better their own position. He suggested the latter. Overall, when individuals were allowed to own their own business and they could profit from it, those owners would figure out how to make their business offer more to the public, consequently making them develop new and more efficient ways to create the things that the public wanted. Greater freedom of economic activity, overall, caused a social good, without the need of preaching to them, threatening them with damnation, nor commanding them from a central location.
So basically the whole core of capitalism comes from one philosophers goal to make the world a better place, where instead of figuring out how to split the pie best, we make more pies.
Now, technically, this sort of works regardless of ethics. Good people work hard to make things that society needs because they are good people. Bad people, in a capitalist economy, will still work hard to make things that society needs because they can get rich doing it and one day retire to their own personal island, with a twenty year old trophy wife/husband.
And it’s always cheaper for a country to trade for the things it wants, rather than to spend a whole bunch of money to invade, conquer, subjugate, occupy, and manage a second country. So, again, simple economics plays into the hands of ethics, regardless of the motives of the actors.
Capitalism results in ethical behavior while never imposing any concept of ethics on people. It basically just tells people to live as they want and to trade with others, with the assumption that people will of their own act in a mutually beneficial way, regardless of their personality.
Others, for example Ayn Rand, have suggested a pattern of ethics to go with capitalist thought. But that’s probably just as silly as Christian or Confucian philosophy, which try to tell people to act like someone who they aren’t, and consequently fall apart. We’re not good, we’re not bad, we’re not selfish, nor are we selfless. We’re just human.
Capitalism was noticed, by Smith, to play into human nature and push it in a positive direction. There was no grand underlying theory. Rather, he noticed what actually happened in the real world, pointed it out, and wrote why he thought that it had that result and suggested that others try it to see if they saw the same thing happen.
Overall, I think that human nature makes us all want the world to become a better place. None of us want to have our children die when they are young, nor do we wish that on others. None of us want to live in a windy shack with no heating, nor wish it on others. We’d all like to have a computer and, overall, we’re quite happy if everyone else does as well. On the whole, humanity is a fairly benevolent force when allowed and, subsequently, once exposed to a functioning capitalist market, societies tend to stick with it.