[mcnees287]

Meaning, the mortgages (default risk) are guaranteed by the government, either explicitly (FHA) or implicitly through a government sponsored interprise (Fannie and Freddie).

After the housing crisis the private market (mostly banks) left very quickly. FHA and the GSEs stepped in and took their share.

[gjem97]

Does the FHA issue bonds? (Turns out this stuff is nigh impossible to Google.)

[mcnees287]

Yes. FHA and the GSEs pool mortgages from banks, stamp them with their guarantee, and then sell the loan pools as MBS to investors.

(Most) banks, especially small ones, are unwilling to sell and hold a fixed rate 30 year mortgage. There's way too much risk, price volatility (driven by rates) and uncertainty about what will happen over a 30 year time period.

Banks only do a 30 year mortgage because they know they can sell it to FHA or the GSEs.