Simply, they "paid into SS" then spent the money on other things they wanted at the time (by "borrowing" the funds from Social Security). Services they didn't want to pay taxes for, but which have nevertheless been consumed, by them, long before I was born or able to benefit in most cases.

Calling it a "social security tax" doesn't mean they paid for social security -- because the money isn't there, it's already been spent on other things. So instead, my generation has to make up the difference. Which I resent.