Actually, if you have ever been on the ocean or another large body of water you can see the curvature of the earth. Look out at a boat on the ocean as it moves away from you
As the boat travels away from you it visually moves up towards the vanishing point at the horizon (at eye level). Once it passes the vanishing point the image becomes blocked by the ocean image that rises up to eye level until the boat finally disappears. Physically the boat isn't moving up at all, it's moving forward.
If you look at the horizon it forms a perfectly flat line 360° around you. If you relocate your position so the boat is traveling from right to left or vice-versa and you're at a distance far enough so that the boat is traveling along the horizon you'll notice the boat doesn't fly up off into the sky nor does it dive down into the water.
The line that forms the horizon matches the contour of the ocean and it's perfectly flat. You can see on the ocean an expanse along the horizon that stretches for hundreds of miles and it's flat.
1) A couple pages ago, I did the math showing why we see what we see at the horizon, accounting for both curvature and for the near 0.5 degree arc of light refraction at the horizon. The vanishing point garbage is nonsense, and easily disproved by a simple picture of a sun setting thousands (millions) of miles behind a ship disappearing over the horizon.
2) At any altitude above the ground, the horizon is *never* at eye-level. You can calculate, measure, and observe the horizon below eye-level. I think you're confusing "eye-level" with "looking at the horizon."
3) The horizon is curved at sea-level. You can measure this; it just looks very flat, and because of earth's size, this is what we expect. It's *not*, however, "perfectly flat." Again, we can measure this.
4) The horizon line you see is also not "hundreds of miles." Not even close. We can measure this.