Because it's notoriously difficult to measure programmer productivity, companies are generally going to be incredibly hesitant to offer "rockstar" programmers salaries that match their capabilities. There's no box office take or album sales or stadium-filling metrics to rely on.

"Rockstar" programmers generally need to go and (co-)found their own companies to make their multiples of millions. The great difficulty is that being a rockstar programmer doesn't always make you a good product/business person, and the best programming can't save a crappy business. But people like Gates and Zuckerberg are the ones who make it happen.

But if one day we find the holy grail of accurately measuring programmer productivity... (though I don't think that's possible...)