[DFHippie]

In addition to the reasons others have cited, I think the corrosive effect of wealth inequality on social cohesion is under-appreciated. The wealthy fear their wealth being taken, so they live in gated communities and hire guards. The society they have walled out and the society they have walled in see their interests as diverging. Neither wants to invest in the other, protect the other. The less wealthy see that wealth buys power, which buys "voice." The government responds to voice, so however democratic their nation is on paper, they don't perceive the government as responsive to their desires -- and in fact it is less measurably less responsive to their desires -- so they have less stake in preserving its legitimacy and stability. Why preserve the tool someone else uses to cheat you? And so on.

Unequal societies are low-trust societies. Low-trust societies are less capable of providing public services efficiently. They are less capable of making and achieving long-term goals. They are more prone to corruption and criminality. If you don't trust the agents of the state to keep you safe, you are more prone to taking matters into your own hands, less prone to cooperating with the agents of the state to address crime. And so on.

https://en.wikipedia.org/wiki/High-trust_and_low-trust_socie...