Stand on a beach. The ocean ends somewhere. That line is not infinite. It is the curve of the Earth eating your view.
The horizon is not where the sky meets the ground. It is where the Earth's curvature hides what is behind it. Standing at 1.7 meters, you can see about 4.6 kilometers. That is it. The person on the other end of a 5-kilometer beach is already hidden by the curve of the planet.
On the Moon, with its tiny radius, the horizon is much closer. Standing on the Moon, you can see about 2.5 kilometers. The Moon looks like a big ball because its curve is sharp. On Mars, the horizon is about 3.8 kilometers. On Jupiter, with its enormous radius, the horizon is so far you cannot even imagine it. You would see 15 kilometers away. Jupiter is so big that the curve looks flat to you.
This is why sailors climbed masts. They were not being dramatic. The higher you climb, the farther you see. The formula is simple: horizon distance = sqrt(2 x height x planet radius). That is it. No tricks. The bigger the planet, the farther the horizon. The higher you stand, the more you see.
On a perfectly flat Earth, the horizon would not exist. You would see forever. But the Earth is not flat. It curves. It hides things. The horizon is the Earth saying: you cannot have everything. You can only see so far.
Because your eye height increases. From the top of Mount Everest (8848m), the horizon is about 337 kilometers away. That is why mountain tops have views. They are not taller than the world. They are just high enough to peek over the curve.
Because the atmosphere bends light. Atmospheric refraction makes the horizon appear slightly lower than it actually is. In very hot or cold conditions, the refraction changes. Ships appear to float. Cities appear on the horizon when they should be below it.
At 10,000 meters, the horizon is about 357 kilometers away. That is why you can see the curvature of the Earth from a plane. The curve is 357 kilometers wide. You can see it bend. From the ISS at 400 km, you can see 2,250 kilometers. The Earth looks like a ball.
Because the Moon is small. Its horizon is close. The curve is sharp. You can see it bend. On Earth, the curve is gentle. On the Moon, the curve is obvious. That is why astronauts on the Moon said it looked like a big ball.