In a democracy the government is inherently an extension of the people and so when you frame it as fair meaning whatever the government defines it as you are, at the same time, acknowledging that the majority have already decided the matter to be fair. Whether or not those people are willing to admit to the world or themselves that they believe the law to be moral, it is from their consent as a mass that the law was able to manifest. So at the very least the law is what most people most people think is morally fair.

In aggregate maybe. But surely there are specific laws which a majority think are unfair.

It’s only the laws which are unfair AND which affect a majority of voters, where you could maybe claim democracy has weighed in on the fairness.

History is littered with laws which are only unfair to minorities, which I think proves my point.

The US is not a democracy by definition, and even if you want to substitute in “republic” here, the description of how the law-making sausage factory works here is idealistic at best.