And doesn't most, if not all, of those reasons come down in the end to greed? Greed for power, greed for material things beyond what is actual need? In short, "want" rather than "need"?

Greed turns people materially inward so that they satisfy only themselves, and this will ultimately consume them. There have been, and are, the wealthy who do Good with their wealth. Unfortunately, they seem to be few and far between.