Because the words temporary amd nonimmigrant don't carry the meaning that you're imbuing into them. Fiance visas operate very similarly to these dual intent H1 visas. You're granted a temporary nonimmigrant status while you pursue a permanent one. The words nonimmigrant and temporary doesn't exclude pursuing a permanent status at all.
In the case of a K-1, it is assumed you will transition from a temporary nonimmigrant status to a permanent status. [1] Requiring folks to move to the U.S., and then go back out of the country to get a green card, only to return again, is absurd. That absurd dance for both K1 and H1 w/dual intents is the reason the laws and guidance provided to agents changed starting in the 50's through the 90's. These changes in guidance to agents are nothing more than a thinly veiled attempt to suppress people coming to the U.S. lawfully, which is absurd and deeply anti-American.
[1] https://www.uscis.gov/family/family-of-us-citizens/visas-for...
The K1 visa is even more temporary! It’s only for 90 days, and under the section 1255(d), the government doesn’t even have the power to convert a K1 visa into permanent residency. It’s like a tourist visa. The visa itself is not a pathway to anything.
People who come here on a K1 get permanent residency once they get married through a different statutory route: 8 USC 1154. But that has nothing to with the K1 itself. That route is available to anyone married to a US citizen, including illegal immigrants under certain conditions, or aliens who get married abroad. The K1 visa isn’t a stepping stone to permanent status. It’s just a convenience that allows people to have the wedding in the U.S. instead of the spouse’s home country.