That is because Social Security payments are suppose to come from a specific funding source by law while welfare come come from the general non specified government funds. And because of increasing lifespans and fewer actual workers paying into social security it must fall, its just math. Not unlike the Ponzi schemes, it just has the advantage that people did not rush in to invest looking for a fast payout so the collapse is in slow motion and is less noticeable.
It also gives the government time to react like it has by raising the age for social security eligibility and putting Federal workers who had guaranteed pensions thus no need for Social Security retirement insurance into the system In effect adding a few more payers now (actually 1984), but then also adding even more receivers down the line so it just delayed the collapse of the Ponzi. They are after state government workers now, who for the most part have the gold plated pensions that the Feds had before 1984 to get their money to kick the can further down the road. Also they are after the Bill Gates money, the earnings over $117,00/year but then that will destroy the image of a base security trust fund and turn Social Security into welfare in people's eyes. With societal changes since FDR's time that may be possible now.