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.
The Macs drove subsidising the moral hazard of fixed rate loans into the public conscious, a subsidy for home owners, political suicide to take away. Better (politically) to rob from a generation or two to pay for reckless low interest rates.
The weird thing is that 5 year adjustable rates are higher then 30yr. fixed. That only makes sense if interest rates will go down over the next 5 years, which seems unlikely to me.
Fixed-rate mortgages are government-subsidized by a range of mechanisms (Fannie, Freddie, FHA, etc)
Adjustable-rate mortgages are not.
As a banker I am perfectly indifferent whether I make a fixed rate loan or a an adjustable rate loan to the borrower.
I look my cost of funds, tack on my spread and that is the price you pay.
If you look at it from the bank's perspective it makes more sense. I got a 5 year and paid it off early. The bank got about 8% of my home value. My friend has a 30 year and the bank will get ~110% of his home value.
I moved to the US from the UK, where mortgages look more like Australia’s, and I still find it amazing you can fix such a low rate for so long here.