Social Security is something that you pay into so it's waiting for you in the future. What is taken off your pay cheque is supposed to be invested on your half by the government and be ready to be paid back to you when you retire (if you get there). It's the safety net that protects your retirement, assuming you worked hard for your life and paid into it.

Any sort of welfare is paid out of the current year's budget as an expense for those who qualify for it.

So to answer your question: You hear it that way because you don't seem to understand how the programs are paid for.

The longer answer is: Combination of demographics (people living longer, baby boomer generation moving through the system), increased cost of living for basic necessities vs wages (inflation) and government mismanagement of the fund create the conditions where the fund may not be solvent after like 2035 at the current rate. Welfare, on the other hand, can never have this problem because it's not supposed to be pre-paid for; it's a yearly expense. That's my fluffty laymen's explanation.