Because the perfect hedge for a debt is to own precisely your liability for that debt. Dollar stablecoins aren't redeemable as bonds, they're redeemable (sometimes, not for tether) as dollars. Using bonds/loans introduces risk and makes the books look 1:1 (mature bonds may pay the full liabilities -- assuming the issuer doesn't default), but actually it turns out to be unregulated fractional reserve banking as people buy these bonds at a discount.
I'm not impugning fractional reserve banking per se here, just the idea that a stable coin backed by bonds is the same as a stable coin backed by the asset it represents.
