I'll throw in my hat.
It's all about dimensions, distance, perspective and the limitations of the 'design' of the human eye.
The size/dimensions of the object you are trying to see, the distance that it is from your eyes, your physical perspective/position in space relative to that object (not to mention 'mental perspective': beliefs, etc.) and the limitations of the human eye.
If you look at an ant on the ground while standing upright, it looks very small and hard to make out. If you bring your eyes down closer to the ant, or the ant up to your eyes, it 'appears' larger and more detail comes into view.
Fleas have fleas. It's a fact. Just because we can't see them with the naked eye, doesn't mean they don't exist.
A hawk can see mice and insects from very high in the sky. This is because its eyes are 'designed' with a zoom/magnification capability. Nocturnal animals can see better in the night because their eyes are 'designed' with better night vision capabilities; more reflective retinas, specialized rods and cones, etc.
The human eye is limited in its capabilities when it comes to distance and light detection, etc.
The horizon is always at eye-level no matter how you orient your body or where you are in space; on/near the ground or high in the sky. This fact, alone, is one of the rock-solid proofs of Flat Earth. I have always found Flat Earth easier to grasp if imagined as a 'Plane' of existence. Forget everything you've ever imagined about space, 'planets', etc. If you imagine a 'dimension' that is Flat, endlessly extending in all directions, it helps a lot to understand how Flat Earth works.
If an object on the ground (vehicle, building) moves far enough away from you, it appears to shrink and rise to the horizon line before completely disappearing from view. This doesn't mean it actually rises or actually disappears. Same with the sunset. Any object in the sky, including stars, that move far enough from where you are located 'appear' to shrink, move downward toward the horizon and, ultimately, disappear. This accounts for the dynamics of how we perceive sunset as well as how the star Polaris appears closer to the horizon when viewed from different points on the Earth.
It has been proven many times that if you take a very powerful optic and look at the horizon you will see objects that were previously invisible to the naked eye. Objects that 'appeared' to move down behind the curvature of the Earth come into plain view, proving they were there all along.
Mt. Everest is approximately 3300 miles from the area of Indonesia that you are referring to. The portion of it that you would be able to see over all the geographic obstructions between it and Indonesia is about 3 miles wide. If you were to use a telescopic device powerful enough to see that far, you would see it. So when you consider the size of that target, its distance from the observer and the limitations of your human eyes, you have your answer.
References to the visibility of the sun and moon can be answered using this same formula. Flat Earth Theory puts those bodies at roughly 35 MILES wide, give or take. So it is clear how they are visible from that distance. Also significant is their complete lack of obstructions between them and the viewer.
{Just a little added nugget to chew on...
(The Hubble telescope technology was not designed for, nor ever intended to be used in, space. It was designed for, and has been used ever since, for surveillance of our world from very high in the sky. One of the well-guarded secrets of the Flat Earth is that it is the perfect shape for surveillance from the sky. With a powerful enough optic, like Hubble technology, you can see every single point on the Earth with incredible clarity, down to the letters on a paper document. You can then record that video feed 24/7, 365 and go back to any day/time and see any event, anywhere, from the moment you began recording forward.)}
I hope this helps a little.