Because we can see too far with zoom equipment.
Consider basic geometry. It shows your assumption is incorrect. I have observed, with my naked eyes. mountains 50 to 60 miles away. The higher you are, the further you can see on a spherical earth. There are limits on the transmission of light through the atmosphere, for various reasons, particulates in the atmosphere, for example. The air tends to be more clear the higher you go, which helps.
Why not?
Parallax.
Why wouldn't you have parallax on a flat surface? For slight curvatures, the difference in parallax between flat and curved is not easily detectable.
When you're on a mountain do you look down to the horizon
or does the horizon come up to eye level?
YOU consider basic geometry
The flat surface isn't moving.