Should we really be upset about that in particular?

It's not at all obvious to me that the more wealth they have on the top end the worse off we are at the bottom end.

It’s about the amount of value a person can bring as a consumer. Does the consumption of food provide more value to society than the consumption of mega yachts?

More equally distributed wealth increases the efficiency of spending (to a point) on the things that society needs to thrive.