Yes, the taxation is regressive, but the benefits are progressive. E.g.,

According to the non-partisan Congressional Budget Office, for people in the bottom fifth of the earnings distribution, the ratio of benefits to taxes is almost three times as high as it is for those in the top fifth.

( https://en.m.wikipedia.org/wiki/Social_Security_(United_States) )

It's certainly not a perfect system, but personally I think it has some merit. And it's by far not the worst aspect of USA tax structure (in my opinion).