Where do you live that you can fix a mortgage rate for 30 years? This is impossible (or at least, highly rare) in Australia (you can get a fixed rate but only for up to ~5 years).
Adjustable rate mortgages exist too but got a lot of people in trouble in 2008. I think a lot of them are only allowed to adjust by a certain percentage regardless of interest rates each year they’re eligible to.
https://www.marketplace.org/2018/10/31/why-do-we-have-30-yea...
* https://www.ratespy.com/best-mortgage-rates/10-year/fixed
* https://www.ratehub.ca/best-mortgage-rates/10-year/fixed
For down payments <20% mortgage insurance is needed (usually from the federal government subsidiary, like Fannie/Freddie), but not for larger down payments: that's all free market.
Some institutions offer 18- and 25-year terms.
Case in point: there is no such government policy in Spain, yet most banks happily offer fixed rates.
You just need to hedge the fixed mortgage you are selling and add some interest to make profit.
It's not just common in the US, it is the de facto standard across almost all mortgage types and sizes. Even weird, non-conforming properties with "jumbo" sized loans can get fixed at 30 years.
Also, I will point out that in both Switzerland and the Nordic Countries, 40 and 50 year mortgages are now in very common use:
"Repayment periods in Switzerland can be incredibly long, with deals lasting between 50 and 100 years being relatively common."[1]
"Swedish regulators calculated in 2013 that the average mortgage term was around 140 years."[2]
In fact, now that I think about it, I am surprised that the US culture of "financial innovation" has not produced more standardized long-duration mortgages ... I do not know anyone, for instance, with a 40-year term ...
[1] https://www.expatica.com/ch/housing/buying/your-guide-to-swi...
[2] https://www.thelocal.se/20160324/sweden-limits-mortgage-loan...