It's too easy to write an annuity as a scam, and there's huge counterparty risk of insolvency over 10-40 years. So we only see annuities where's there's no meaningful choice whether to opt in (Social Security and pensions)
EDIT: it appears that Vanguard itself offers annuities, but they're not particularly transparent so I'm unsure if they're "good enough" for general-purpose retirement savings.
The U.S. government already sells 30-year Treasuries and savings bonds [1]. Spinning up a new government program is complicated. Just authorize the issuance of 50, 70 and 90-year Treasuries.
[1] https://www.treasurydirect.gov/indiv/products/prod_eebonds_g...
But maybe that's how it has to be, since it depends on the person? In any case, this will make them harder to sell.