I remember wrestling with this in my therapist's office when Aaron died. I had known him tangentially - we hung out in the same IRC channels, and had several mutual friends in the Cambridge/Somerville techie crowd that he would hang out in person with.
As a college student and young adult I had always envied his fame, his intelligence, his money (post-Reddit acquisition), and the strength of his convictions. And yet, in that moment in early 2013, he was dead, and I was working a good job at Google (and this was 2013 Google, when it was still a nice place to work doing things that I could generally approve of). And he'd died doing the stuff that I wanted to do but had been too chickenshit to actually carry out.
I think that this illustrates why the world is the way it is. All the true altruists are dead, killed for their altruism. It is adaptive, in a survival sense, to think of yourself and your own survival and not worry too much about other people. Ironically, this is what my therapist was trying to get me to realize.
But I think this also goes back to the GP's point. When people at wealth level x give to people at level x-1, it doesn't raise the people at x-1 up to x. It brings the person at x down to x-1. There are more people at x-1 than x, after all; you could give everything you had away and mathematically, it would lower your net worth significantly more than it would raise theirs. And of course, it doesn't do a damn thing about the people at x+1. Why can't they donate instead, where their wealth would do an order of magnitude more good?
There actually do exist people who are like that: they would rather spread their wealth around the people at wealth level x-1, joining them at that level, than raise themselves up to x+1. I've met some; most poor people are far more generous than rich people are. That is why they are poor. But then, it doesn't solve the problem of inequality, they just disappear into the masses of people at level x-1.