“Because I say SO!” you patiently explain! ;)
There are no obvious and fundamental differences between that tax and any other - other than that it is the most regressive tax out there - one paid by working people.
And yes Virginia, the identical argument you are making can be, and is, made - in regards to any tax cut of any type ever - I hear Democrats making that argument in regards to every proposed tax cut.
It is a regressive tax, but then low income people get much greater benefits per dollar of FICA tax paid than people with higher income. My wife paid about a tenth of the FICA tax that I paid, but her Social Security benefits payments are about a third of mine. When benefits are considered, the Social Security system as a whole is progressive.
Google the words: social security benefits progressive. Look at the chart in the first PDF document (Is Social Security Progressive) that comes up in the search.