Please, before you go, would you mind answering my question?
If the horizon is the curvature of the Earth, and you can clearly see it, why can't you clearly see the curvature of the horizon without being 100k feet in the air?
I'll give it a shot if you don't mind.
First of all, it's easier to notice curvature that is parallel to your line of vision, rather than perpendicular. Imagine going to the hardware store and picking out a long board. You want a perfectly straight board. If you set the boards down horizontally on a bench, and then step back, they might all look fairly straight. But if you hold the board up to your eye as if you are looking down the barrel of a gun, you can see the tiny imperfections. This is because all the tiny imperfections are compressed into a tiny section of your vision, which makes them stand out.
But there is another reason why horizontal curvature is difficult to notice. Mathematically, the horizon is the same distance away from you in all directions (assuming symmetrical terrain). This is true for both a flat earth and a round earth. Therefore, the curvature of the horizon is due ENTIRELY to the dip in visual angle to the horizon. (d
g in this image:)
There are 2 things about this that you should understand:
1. There is a dip in the visual angle to the horizon for a flat earth as well. Unless you believe that the horizon is an infinite distance away, which most flat earthers don't believe, for obvious reasons. This means that there should be curvature for a flat earth horizon as well, although it would be slightly less than that of a round earth.
2. How you perceive this curvature is entirely dependent on how the 3d view of the horizon is projected onto a 2d image. For example, for a cylindrical panorama, the horizon would appear perfectly straight, regardless of the dip angle. Think of the lines of latitude on a globe. They are curved, right? But in a mercator projection, all lines of latitude are perfectly straight, regardless of "dip angle" (degrees away from the equator).
If you want to get a feel for how much curvature you would see based on a given "dip angle", I recommend downloading
Stellarium.
1. Turn off the ground and atmosphere.
2. Turn on the Azimuthal grid.
3. Position the camera so that the horizontal line labelled "+0 degrees" is in the middle of the screen. It should be perfectly straight.
4. Zoom in until your horizontal field of view is similar to the average camera. (about 60 degrees)
5. Now look at the curvature of the line labelled "-10 degrees".
See how little curvature is visible in that line? For reference, to actually achieve a -10 degrees dip angle to the horizon, you would have to be 320,000 feet high. 60 miles high.
The dip angle for a person with an eye level of 6 feet is only -0.04 degrees. That is a tiny fraction of the curvature of the -10 degree line.
Feel free to change which projection Stellarium uses. It comes with a long list of different projections. The one it uses by default is fairly close to human vision.