You make a few grand statements with no backing (e.g., who said Gates decided that Microsoft wasn't important? I'd be a pretty happy person if I'd have contributed as much to the world as MS has).

I can't say for sure what Gates had in mind, but I can extrapolate from the few cases I do know about (myself and other entreprenurial-minded friends). The reasons vary, but I for one am definitely in this to make money for the power to do more important work later.