'rally' ? :)
That's a very tough question, to which I'm afraid there are no valid short answers, unless they're in the negative.
Everybody that wants to change 'the world' has a different definition of 'world'. For some it means their personal environment, for some it means those below the poverty line. Almost always there is some 'demographic' limitation to what the world really means.
I think plenty of people genuinely want to improve the world, but because they realize their limitations they're forced to scale down their ambitions a bit and then they do their best within that framework.
And some get lucky and make a bundle, then spend it to materially improve the lives of as many others as they can manage. But that's rare.
For some people who made a lot of money in not-so-nice ways doing good is a way of atoning, or even PR. For others it is what comes most natural to them. Altruism doesn't have to come in competition with greed, it can come after it, or before, gathering money as a tool. There are all kids of scenarios here.
Over the years I've found that my teen-age idealism ("change the world for good on a large scale") has bit by bit been whittled away by reality. Right now I'd settle for my definition of 'world' to be those people that I'm in daily contact with.
And yes, I really do want to do that, but I'm not sure if I will succeed, for any definition of world.
But I'll keep trying :)