Because I find it fun. If you don't, that's perfectly cool, too.
Others: they ave 2+ kids, have been through many IPO/M&A debacles and may have gone through ridiculous divorces, major swings in their marketability or simply been screwed over by certain companies...
They hype around making tons of money can be subdued by the realities of life in between these relatively rare miollionairing events.
Thus, qualities of work other than "working myself to the bone because this company will be the next google IPO" become a big factor. This is why HP, Yahoo, MS, and others continue to attact talent - although it might represent an aging trend.
There will always be more wily generations in the tech industry. Learn, earn and avoid smugness.