The 30-year fixed mortgage (the entire amortization period is at one fixed interest rate) is a government created product. It would never be offered by banks, at the low rates they are, if they had to hold them on their books. The "normal" mortgages in the US prior to the FHA were 50% down and 5-year terms.

https://bebusinessed.com/history/history-of-mortgages/

Fannie and Freddie basically agree to "back" those new mortgages (typically buying them from banks and selling them off later) as long as they conform to certain requirements - 15 or 30 year terms, 10-20% down payments, etc.

It's a massive subsidy for the housing market in the US, but also addresses the social goal of making housing accessible. You can get a mortgage and the monthly payment (interest + principle) never changes over the 30 year life of the loan.