Our tax policy is among the most progressive in the developed world: https://www.theatlantic.com/business/archive/2012/02/us-taxe....
The real problem is that the average income earner doesn't pay enough Social Security taxes to cover what they will receive in benefits: http://www.politifact.com/truth-o-meter/article/2013/feb/01/.... The system is set up so not only do the needy get a public subsidy, but most people do.
Caps on retirement taxes are not specific to the U.S. Germany and France, for example, have similar ceilings. Germany, for example, has a 19.6% payroll tax up to $80,000. The U.S. tax is 12.4% up to $127,000.