That's a BEAUTIFUL photo. I'd like to know where it came from.
The answer to the question is:
For the same reason that you can see farther when you're higher. Because the Earth is round, a sphere, the shape of a ball.
This beautiful photo is the answer to anyone who claims that the Earth is flat. It totally debunks every possible weird and twisted explanation they can come up with. This picture can't happen on a flat Earth, no matter what you try to do with density, buoyancy, perspective, refraction, sheep, holograms, indoctrination, cognitive dissonance, or NASA.