Well, SS is by definition a tax on earned income. It is not levied on interest income, nor on stock dividends.
It's kind of crummy, when you are taxed on income that you sweat and toil to get, but not on income that comes from your capital while you surf the Web and BS on a retirement forum. But I have been enjoying this, so why do I bring this up? As Orwell said, some animals are more equal than others.
At some point, when they need to beef up SS funding, they may tax everything. Then, we retired people will pay SS on everything. See what you think about that!
Now, will they do SS tax on SS benefits? Heck, why not?