Is that an answer to Why not? Are you suggesting it's locked up because the government can't pull all that money out of US debt without causing ripples?
Otherwise, while US debt has historically been stable, the returns suck.
There are laws that govern how the "trust fund" can be invested. The laws were not designed to chase returns but rather to ensure solvency.
We are now in the draw down phase... the fund is de-accumulating slowly. There's no reason to "pull all the money out" because most of the payout obligations are long-term, but the small amounts each year that need to be converted to cash must come either from budget dollars or from new debt sold to outside buyers. That's a big change from the years and years of borrowing
from the trust fund.