Financing at 3 months currently using SPX box spreads is around 4.45% (anualized) [https://www.boxtrades.com/SPX/18DEC26](https://www.boxtrades.com/SPX/18DEC26)
However, the 3-month US treasuries trade at 4.087% [https://www.investing.com/rates-bonds/u.s.-3-month-bond-yield](https://www.investing.com/rates-bonds/u.s.-3-month-bond-yield)
So if I want to borrow money at the best rate it seems a better deal to short US treasuries than using SPX box spreads.
What is the catch? Why this diff?
Anyone tried shorting bonds as a way of financing?