That's how they were sold to the public. In reality, they are taxes, and the government has no legal obligation to honor the promises of payment. Hence, those who are in line to revceive payment from the government, from those programs, have a personal interest in having them preserved even if the programs are financially non-viable.
Not saying the programs are non-viable at this moment in time, but they are in the long run. That's why Congress keeps tinkering with eligibility.
Social Security and Medicare are pay as you go programs, i.e., today's workers pay for today's retirees. Medicare has been running in the red since 2008 and Social Security has been paying out more in benefits than it receives in revenues since 2010. As a result, to make up the shortfall, the General Fund must come up with the money (40% borrowed) to redeem the non-market, interest bearing T-bills in the HI, SS, and DI trust funds to pay full benefits. The DI Trust Fund runs out of T-bills in 2016, at which time benefits must be cut by law. The Medicare Trust fund will last until 2026 and SSTF until 2033.
The reality is that these programs are not viable now as currently structured. We are borrowing money to pay benefits now.