Oh it is explained, though, but not consistently. I have heard explanations of that the light refraction makes the sun look lower when it moves away, to a point where its visibility is hidden by the horizon, like a mirrage of the sky. Another explanation is that the surface of the earth isn’t flat, but bulges slightly, like a dome, making the sun disappear behind the dome every day. I’m sure there are more explanations as well.

The thing about flat earth theories is that they always have an answer for whatever you throw at them. They are so committed to their idea, that they will invent reasonings on the spot which become more and more convoluted - often in conflict with other parts of their theory.