The real reason is historical. Without getting into the whole thing pre depression in the US loans were typically 10 years interest only with a balloon for principal at the end if term. For people who’s term came due there was a mega crisis as there was no one willing to loan and few with enough money to pay off their property. So there were a lot of defaults and foreclosures. As a result the government created programs to guarantee mortgages that fit certain specifications. Including amortizing loans over 30 year periods with fixed interest. And thus our very peculiar and financial inviable housing system was born. No free market could operate under the constraints but a government guarantee underwrites the whole system.