> I know these people exist everywhere today but there's an unusually large number concentrated in Chicago. Must be something in the water?

Finance is also an extremely tempting lure. The pay is quite good (e.g., significantly better than Google), the hours are better, and the firms are generally supportive of external work. Many of the finance firms my friends work at (I've lived in the Chicago area most of my life) allow/encourage/hire people to serve on the C++ standardization committee, were co-founders of major open source projects such as R, etc.

It's hard to lure people away from jobs like that where you're paid a lot and have a ton of freedom for a potential exit that's smaller than what you currently make, unless your startup becomes Google.