Social security is a mandatory insurance program, not a mandatory investment program. Like any other insurance program, the solution to shortfall is to increase premiums or decrease payouts.
Since there isn't yet a shortfall that can't be covered by existing funding sources, these solutions have not yet been implemented. Just like a private insurance company.
Social Security is a ponzi scheme. Insurance companies have assets the can fall back on to pay claims, they don't just pay them out of insurance payments as they come in.
What? Insurance companies primarily finance the payment of claims from premium payments. Doing anything else would indicate an unacceptably high loss ratio and a failing business.
Since there isn't yet a shortfall that can't be covered by existing funding sources, these solutions have not yet been implemented. Just like a private insurance company.