Because people confuse “fair” as in “moral” with “fair” as in “no rules were violated”. These two are very distinct meanings, but a confused mind jumps between the two at its own convenience, picking the one best suited for the argument at hand.

Capitalism is a fair game. So if it’s played by the rules, any outcome of it, including massively skewed distribution of wealth and power, is also fair in this sense. If no rules were broken, any outcome is according to the rules, so it must be fair, too. This is good logic, and this also is the logic of libertarians.

But this just means you don’t question the rules themselves, you take them as God’s gospel. You don’t, for example, question the rule that allows individual people to own institutions. Or generally to play the game without regard to others’ well-being. It just means that the rules of the game are above all else for you, and it’s callous.

It’s easy to see. Imagine we’re a family and we set a rule that everyone eats according to the effort they put into family work. Fair (as in “moral”)? Fair. Much fairer than capitalism, actually, because capitalism, stricly speaking, doesn’t care about effort at all, only about outcome. Effort is inconsequential in capitalism, the only thing that matters is whether you succeed or fail. But we are a family, so we’re more humane. So far so good, very fair. Now a new child is born, and he’s disabled. He can extend no effort to feed himself. His effort is zero. According to the rules of the game, he must be starved to death. Fair? Fair. No rules are broken. Have you noticed the bait-and-switch?

If you just assume that the system is moral in its foundation, then it necessarily follows that any outcome of the system is moral, too, and you will defend it. The only way to stop seeing it this way is to reconsider your assumptions about the system, and you can’t do that while your only focus is to ascertain whether the outcomes were produced fairly within the system or not.

That’s why it’s very hard to argue with defenders of capitalism, it’s like religion. Whatever you point out they look at the context of whether any rules were broken or not. But they never question the rules.