1st Flat Earth is to be considered on the base that when they did that the last time a dimension of travel was not known.
We can travel by air for only around a hundred years , with mind you 2 world w ars , meaning no time to look around for the
common man.
2nd My idea is that the church had to give in to round Earth to stop people trying to make the next leap to a dimension.
If they figured out long sea travel and in a short time they found a NEW WORLD as big as the known world... what would happen if you make the jump to a dimension ( AIR TRAVEL , SPACE TRAVEL ) and find a BUNCH of new worlds?! We are probably fucked... we will have to conquer for days... upon days... its too much work.
3rd People use geometry Euclidean Geometry to measure the world... but they use a
not real geometry to explain round Earth... THINK ABOUT THAT