To quote Samuel L. Jackson's line in Pulp Fiction ... "Allow me to retort."
1. Because the government is inconsistent on taxation. I can (and do) take after-tax dollars and invest them in stocks with no dividends i.e., no tax drag. 50+ years of untaxed growth and then heirs get them and pay NO taxes after the step-up. Rinse and repeat. Or ... late in life, you need to withdraw those funds, you're paying taxes on the LT capital gains at 15% versus the last-to-die paying a likely 24% on 85% of a SS payout. So, I'll put your own question back to you ... why should SS be any different?
2. As others have mentioned, that looking strictly at your SS taxes paid, ignores the cost of money, which becomes quite considerable when considering many of us first put into the system 50 years ago (before taking a dime out). This also ignores the company's contributory payment. That's not imaginary fairy dust.
3. As others have launched on ... not indexing income thresholds puts more citizens in the 50 & 85% tax threshold each and every year and is a flat out stealth tax increase.
SS was sold as a non-taxable benefit, which was true for almost 50 years before first 50% and then 85% taxable levels came into play and stealth tax increases above that. So, there is a BIG difference between understanding that SS is a double-taxation system that affects more and more retirees every year and just acknowledging that (while SS has served many well) that it comes at a cost and those costs are increasing.