To be fair, they don't "dip into" it. They "invest" it. In conservative, low-risk, fixed-income investments. Like Treasury Bonds.
Which is why "lock box" was the biggest load of hooey: it basically was Gore saying "We're no longer going to purchase US debt with Social Security funds", which was never going to happen. No one would just allow cash to sit around waiting to be disbursed, not earning any interest. They would have taken the cash out of the "lock box", bought Treasuries, put the T-Bonds back into the "lock box", and bragged about how smart they were for growing Social Security money in perfectly secure investments...even though they weren't doing a damned thing differently than before.