In ancient times, people did not have the concept of a "civil day"; they viewed day and night as separate things which alternated.

The Assyrians divided the day into six equal parts, and the night into six equal parts, called ush. But because sunrise and sunset move around over the course of the year, the lengths of day and night vary, and thus one ush during the day would not be the same length as an ush at night except around the equinoxes.

The Babylonians divided ushes in two to make hours, because it was easier to do astronomy in 12s than 6es. This resulted in 12 hours in a day and 12 hours at night, but daytime hours were still different lengths to nighttime hours, and the lengths of hours still varied over the course of the year.

The Greeks partially adopted the Babylonian system; they divided the day into 12 hours but the night was divided into four watches. The Romans copied the Greek system, but later went full Babylonian with 12 hours at night as well. (I feel like this coïncided with the rise of Christianity, but I have no evidence). The Romans introduced the concept of the civil day beginning at midnight (which the Chinese independently came up with), and over time, this led to the idea of 12 hours from midnight to noon, and 12 hours from noon to midnight. That idea postdates Rome, however; Roman hours were reckoned from sunrise to sunset and sunset to sunrise.

Assyrian astronomical knowledge seems to have reached China via India, as traditional Chinese timekeeping divides the civil day into 12 shi. Ancient shi were like Assyrian ushes; they were either 1/6 of a day or 1/6 of a night. Originally, midnight and noon fell in the middle of a shi, but this was changed to shi starting at midnight to make administration and astronomy easier. This system of variable-length shi continued to be used in Japan until about the Meiji Restoration.

Fixed-length hours are the result of analogue clocks, which are impractical to design to change the lengths of hours with the seasons (but not impossible; the wskusei clock is an ingenious Japanese clock from the 17th century that does exactly that). China had reliable, accurate water clocks by the Tang dynasty, while Europeans developed circular mechsnical clocks in the late Middle Ages. In neither case was it practical to make something as clever as the wakusei clock, so analogue clocks were marked the mean length of a shi or an hour as a reasonable approximation. Since there are 12 hours from midnight to noon and 12 from noon to midnight, that led to the 12-hour time system we know today.

You've never seen an analog clock?

That's where it comes from, and that's where it makes perfect sense. (An 8 hour reset wouldn't) No need to carry that over to digital clocks however.