It's not a massive optical illusion, it is limited perception. When you look around over "flat" ground or water, even on a clear day, how far out can you see? 50km tops? Translated into RET, that's not even 0.5° of the Earths curvature, certainly imperceptible to the human eye, even if you were standing on a perfectly featureless surface. Sure, you can see mountains at greater distances, but they don't exactly count as "flat" do they?
I know you're going to keep arguing this point because it contradicts one of the fundamental observations of FET. It should be clear that an extremely small portion of a sphere will appear flat. If I had a metal sphere with a circumference of 1000 meters, then shaved a 10x10cm square of the surface, that square would obviously appear to be flat (0.0036° curvature). I cannot provide more conclusive proof that an extremely small portion of a sphere will appear flat, because it is a matter of perception.