Easy: they appropriate a greater portion of humanity's productivity towards themselves. Wealth is power, the ability to express desire and recruit human effort to its fulfillment. When the rich have all the money, the poor cannot express their own desires and end up just working to satisfy the ends of the rich.

Can you give a concrete example? Something that I can't do now with all these super rich people around that I could have done when they weren't so much richer than me. If I'm competing with them to buy a Picasso, sure, they'll outbid me, but what about things I actually want to do, like buy equipment for my hobbies or go travelling. Are they making that more expensive?