I'm not about to second-guess PG, or any other single person. Maybe the capital they have isn't liquid enough; maybe they have other obligations; who knows?
And it's worth pointing out that doing more awesome things doesn't require wealth. Why don't more poor people clean up their neighborhoods? Why don't more startups choose to solve problems that would really improve peoples' lives?
We should never point our fingers at anyone else before looking hard at ourselves.
All that said, the accumulation of wealth tends to require a mindset change that is not conducive to working on awesome things unless there's a reasonable chance of those awesome things making even more money, in the future if not immediately.
For one example: I recently moved in to a house, which I'm renting, which was a real dump and eyesore in its neighborhood. It was located next door to a nice Victorian bed & breakfast; most of the places in the neighborhood were a lot better by comparison. In the last 7 months I've completely transformed the yards, repaired or replaced plumbing, put interior walls into the garage -- all out of my own pocket -- with the help of my friend & my girlfriend. We often get comments from others in the neighborhood; those with more money tend to ask us if we're planning to buy it, and those with less money tend to just express appreciation that we're doing it.
So there ya go.