If you were paying retired people with investors money then why does SSA have a giant surplus of nearly 3 trillion [1]?

The surplus is because of all the people that have payed into the program and haven't retired yet ...

[1]: https://www.ssa.gov/oact/progdata/assets.html

If only they just used the current 'investors' money to pay the retirees. They actually use the social security taxes to pay for "whatever" and hope they can come up with the rest when they need it.