For one, people's expectations change. I couldn't count the number of times I've seen a Bay Area software engineer 4 years out of college genuinely believe they're justified in complaining about their lot in life because they can't afford a 5 bedroom house in Palo Alto, all the while being driven around and being fed for free by workers making $20 an hour who have to try and live in the same area.
Look at the obvious internal strife at places like Google over efforts like Maven and Dragonfly. If it was just money motivating them, would you see that? Would you see people who leave over it?