You are relying on Sight for present information but when you add MEMORY you construct a logical Earth that is a sphere.
I have been to NYC a couple times. I have a Memory of it yet in my Present I couldn't honestly tell you NYC still exists. My furthest sensor stops at the horizon.
The Flat Earth is instruction to live in the present and not manufacture castles in the sky from memories of events past.
Right now my Earth extends from Mt Graham, the Chiricahuas, Swiss Helms, Agua Priest, the Sulfur Spring valley, Whitewater Draw and the Dragoons. You might try to tell me something exists beyond but that would be hearsay and I couldn't testify to it