It looks that way because the earth isn't flat. On a flat earth, again, the sun would never disappear over the horizon. Ever. That is literally impossible. If you roll a marble down an infinitely long flat plane, the marble never dips below the plank. That would make it a ghost marble. Similarly, an object circling above a plane can never drop below it. (Obviously it could disappear behind a feature like a mountain, but never the ocean.) Even visually. It would simply shrink in a spot (very close, but still) above the horizon.
It looks that way because the earth isn't flat. On a flat earth, again, the sun would never disappear over the horizon. Ever. That is literally impossible. If you roll a marble down an infinitely long flat plane, the marble never dips below the plank. That would make it a ghost marble. Similarly, an object circling above a plane can never drop below it. (Obviously it could disappear behind a feature like a mountain, but never the ocean.) Even visually. It would simply shrink in a spot (very close, but still) above the horizon.