I was interested to hear a rational the explanation for night time with a flat earth scenario. Are we saying it's because the sun was too far away in the sky?

We know the further away an object gets, the smaller it appears. The problem with that is the sun maintains a diameter of a 0.5 degrees regardless of where it is in the sky. This can be confirmed by anyone with a pair of solar viewing glasses. It indeed maintains that diameter all the way to the horizon, which is at a point it would be at the most distant it could be in a flat earth scenario.