Sunrise and Sunset refers to the time of the day when the light of the Sun rises from or sets into the horizon. The Round Earth model describes sunset as a rotating spherical earth that obscures the Sun. The Flat Earth model describes sunset as the light of the Sun setting into the Flat Earth.
In accord with what we see, a rotating spherical Earth which obscures the Sun is not seen. We do not see the horizon rising upwards into the Sun. The most direct description of what we observe is that the light of the Sun is setting into a Flat Earth.
Thus established that the most natural interpretation is that the light of the Sun is moving and setting, a mechanism which describes how and why this occurs may be found below.