[hapless]

Fixed rate mortgages are subsidized by the US government, that's why. The mechanism of the subsidy is extremely complicated, but it is not a small effect.

Before the creation of the enormous state-owned insurance corporations and government programs to drive down those fixed rate mortgage costs, American mortgages were usually short-term, with giant balloon payments. Those short-term, balloon-payment mortgages went bust in huge numbers during the Great Depression, creating pressure on the government to "do something."

Say what you will about American housing policy, but those 15- and 30-year mortgage arrangements are very stable.