Edit: Answering my own question the treasury sells EE Savings Bonds the can be redeemed early, and T bills that can be sold to someone else and not redeemed by the US government.
Clearly the government would like to lock in rates expected to be as low as possible for as long as possible.
Check out projections of US interest on debt expense. In a few years time it may exceed all other spending categories. Much of the US government borrowing is short term.
What happens when rates rise? So, it makes sense to lock in as much borrowing they can at low rates.
I don't think that's how T-bonds work. You can sell them on the secondary market, but that doesn't matter to the government — it locks in the rate it sold them at.