Kathy Ruffing explains them:
[A] new analysis by the Center for Economic and Policy Research (CEPR) confirms that means-testing would yield very little in savings … unless we took benefits away not only from rich retirees, but also from many who are solidly middle-class.
The reason, as the CEPR analysis shows, is that there aren’t enough rich retirees and they don’t collect enough in Social Security to make much of a difference. Only 2 percent of Social Security benefits go to retirees with other (non-Social Security) income of $100,000 or more each year. ... Only about 10 percent of benefits go to people with outside income of $40,000 or more a year a figure that most of us would regard as middle class.
So let's see: we could reduce social security outlays by ten percent without harming the poor and needy. And we could reduce them even more by gradually extending the retirement age. Some combination of the two could bring the program back to long-term solvency. So why not?