Why do you think that was allowed to happen? Hint; it was designed to be that way from the beginning. The government presented it to the SCOTUS as a tax which means it is revenue. Congress can do whatever it wants with tax revenue, and they did. BTW, what you wrote is partially true, but even that would not make SS sustainable, it just would have lasted longer before going belly up. It would be a better funded ponzi scheme. Many people who paid 6% of their $10K annual wage in the 1970s into the system are now getting more than that back every month in benefits. That does not sound sustainable to me, but what do I know?
It was designed to keep elderly people from dieing in the streets from exposure and starvation which is what was happening prior to its inception. It’s primary purpose was to be a social safety net but it eventually became an indirect extension of the general tax fund. The underlying problem with social security is unemployment and underemployment which have led to decreased economic growth. This decreased employment/revenue/growth coupled with the inflationary nature of fractional reserve banking/federal reserve banking has formed something of a perfect storm; however, regardless of the government’s capital mismanagement, I have been paying into social security for over 20 years and the government owes me some money (me and millions of other Americans). This is not an entitlement, it’s more of a contract and this is, of course, what makes social security the third rail of politics.