Or it’s because it’s politically popular to raise speed limits since they usually raise them to the speed that is almost, but still below, the speed that everybody is already driving at currently.
Also in the US the AAA heavily invests in lobbying for higher speed limits every year. The AAA isn't quite the villains in American politics that the NRA are (ETA: or at least, have become), but they are historically close. It's easy to forget that AAA was founded to fight speed limits in the first place.
I'll admit it is something of an uncharitable view. The given reason AAA was founded was to improve the "quality" of roads in the country. One of the qualities that was seen as lacking at the time was the inability to use anything close to the top speed of cars due to road hazards and pedestrians. It's related to why AAA was the first car racing body in the country, overseeing for instance early versions of the Indianapolis 500 (though it divested racing later on).
(ETA: The search term I was forgetting was "jaywalking", because AAA was instrumental in the creation of jaywalking as a crime, related to the goals of allowing faster car travel versus speed limits. https://www.salon.com/2015/08/20/the_secret_history_of_jaywa...)
