Who decides what's "fair"? Shouldn't the market decide? Otherwise, of course people will always be incentivized to argue that their subjective opinion is what's fair, and that's almost always going to be, "I should have been given more."

I don't have an answer for that, again I don't subscribe to that belief and I think risk should be highly rewarded.

That said the market is a terrible judge of fairness since it's a feedback loop so luck early on can definitely allow you to extract an unfair amount of value later(this is what Wilkinsons employee was saying basically, Andrew pointed out that they offered them share packages instead of salaries and the employee replied that his employees couldn't afford to live without a salary like he could so it was never a real option so they didn't actually have a chance to capture a fair amount of the value from their work).

(It's been a while since I read the book I could be getting some of the details slightly wrong)

The market is not some independent external arbiter. The market is based on rules government creates. Those rules can be exploitative - It may be perfectly legal to make payday loans to financial illiterates, or treat animals cruelly in meat plants, or simply to use marketing in a way to manipulate people's behavior subconsciously (which almost every company does).

Everyone who makes a billion dollars must attribute that success to themselves, but also the people who built the roads their product is delivered with, the teachers that taught them in school, the doctors that keep them healthy, etc.

What fraction of the wealth earned by a billionaire society is entitled to for its contribution will always be a fair policy debate.