@stevecanuck, I think I get what you are saying, and yes, from our perspective as being tiny specs on a huge surface, we literally cannot observe curvature directly with our own two eyes.
I think you missed my point, so I'm going to try again.
Imagine being in a row boat in the middle of the ocean. Now look in any direction and you will be able to see approximately 2 miles from a sitting position with your eye level at 2'6".
If you turn in a full circle you will therefore be the center of a circle with a radius of 2 miles. Every spot on the horizon will look the same to you and will therefore appear to be flat.
Or, hold a hula hoop over you so it's parallel to the ground and at eye level. As you turn 360 degrees you will see the edge of the hula hoop at exactly the same level above the ground and at eye level height, it will be a flat plane.
So, we ARE seeing curvature. It's just on a horizontal plane and NOT down to the left and right. You're looking at the hula hoop from the middle of it looking outward, and NOT from the side with it being held vertically.
I hope that explains it. Bottom line is that it's physically impossible to see left to right curvature because, you would have to see farther to the left and right than you would have to see in the middle.