In my experience, maybe 2% of software engineers are incentivized more strongly by positive social change than by money or fame. These are the idealists, like Richard Stallman and former members of OLPC, who are often ridiculed by the rest of the community. Until the academic and governmental infrastructure is able to put anywhere near as much money into CS research as private industry, the majority of technologists will (have to) be driven by the desire to make money.
For all the talk of how doctors make a lot of money, the reason they do is that so few are both qualified and willing to take the long, grinding road through 8 years of medical school and residency, $200k in debt, and 80 hour work weeks, with fierce competition at every step of the way. According to the med students I know, if you aren't inspired by some higher calling it's almost not worth it because you won't make it through.
