I'm one of them. If you can excuse my lack of humility, I am a great programmer. I've been described by management and teammembers regularly as a rock star. I could easily make 10x the salary were I to move to SV.
I have zero desire to and I've never felt like I was missing anything. Out of my graduating class, only one guy moved to California - in fact, he's the only guy I know of at all who moved to California. I know at least a dozen "great" programmers who live here quite happily.
Of course the extra money is an incentive and there obviously has been and is an influx of programmers to Silicon Valley and the surrounding area. But the wage impact (in terms of setting floors) of the migration is pretty minimal relative to other effects. That's why there are hundreds of thousands, if not millions, of programmers worldwide, including very close to the full 5% of "great" programmers from those areas, that are working for what you and I would consider garbage wages.
This mobility checks salary bottoms for those who do want to stay (e.g. for the market I'm in over in China).