One thing that is very discounted in the OP is the government. Many very good programmers work in the DOE, DOD, and other 4 to 3 letter named departments. Programmers, like the rest of us, are very diverse people. SOme like the 'fun' you talk of, some really value the stability of a mega corp or the CIA. Some are there only for the problems and their personal interest in them, some are there for their kids and kiss off the desk at 5pm exactly. Some like to paint, some like to go to Burning Man. Programming talent and 'rockstar' status is independent from motivations to do a job I think.
Also, the assertion that most top talent work at mega corps is a statistics issue, as other comments mention. More jobs, as a percentage, are at mega corps and the government. But more companies are under 100 to 50 employees. So, if you look at it from a percent of programmers, you see many more at mega corps. But as a percent of top talent, you see more at small businesses, simply because you can determine the top talent easier at a start-up as there is less debate who the top people are. Other commenters explain this better, sorry.
Also, the assertion that most top talent work at mega corps is a statistics issue, as other comments mention. More jobs, as a percentage, are at mega corps and the government. But more companies are under 100 to 50 employees. So, if you look at it from a percent of programmers, you see many more at mega corps. But as a percent of top talent, you see more at small businesses, simply because you can determine the top talent easier at a start-up as there is less debate who the top people are. Other commenters explain this better, sorry.