If you look into bend points, you will see that the first amount you contribute gets you a significant return later for your SS check, but as you contribute more, the slope of the size of the SS check flattens. After the second bend point, adding more into SS doesn't get you a much larger check.

The reason the rich wouldn't want to further contribute then, is because at that point, their contributions are getting a very poor return, and they would feel they could do better on their own. Since it isn't a tax, they would argue (correctly) that it is a waste of their money compared to investing themselves.