A missing piece of the puzzle may be regulatory capture and a strong political/legal structure that resists the worst ambitions of cruel people whether they be wealthy or poor.
You can think of wealth like the potential energy of a spring under tension. If used properly it is capable of powering the most amazing and intricate social mechanisms but if poorly regulated it destroys social fabric and the well being of every day people.
Things like Citizens United and lobbyists representing cruel wealthy interests running unchecked over American democracy are examples of the socially destructive potential energy of wealth.
I'm also curious if there's a selection pressure in play where the more cruel wealthy people in the Nordic countries move to the US because they see more opportunity to make money and be cruel in that environment while wealthy people who have some affinity with their nation and the people of it choose to remain and don't or can't lobby for terribly antisocial policies.