why dont we call in the old bonds and refinance them?
US Treasury bonds don't have a call feature.
"We?" Are the Feds posting here?
The bonds may not be callable, but I don't suppose there is any kind of indenture beyond the "full faith and credit" of the U.S. that would prohibit repurchasing the bonds on the open market via a committee of sorts.
Congress is in control of both spending and revenue. The Federal Reserve and Treasury borrow at auction what they need to finance the debt and no more.
Unless Congress raises taxes and/or reduces spending, why would the FOMC or other government banking agencies engage in any significant repurchasing of bonds, only to reissue as new series?