Pondering Optics and Curves
As things move further and further away, they get closer to the horizon. They also compress, and appear far smaller. This continues until they vanish entirely, due to an angular resolution too small for our eyes to see. This can be proven with powerful optics which bring objects back into view. They haven't disappeared behind an imaginary curve.
BTW, to give an idea of how big those clouds can be, cumulous clouds can reach up to the jet stream at over 30,000 feet. And yet, there they sit almost as if on the horizon.