> Any human who has watched ships come in to a harbor would be able to trivially tell you: We see the masts before the ship.
Of which, until recently, there were very few. Civilizations developed not just on the coasts, but along the rivers, and until ~industrial revolution, the bulk of people at any given time didn't really have a chance to see the sea.
> The only way anybody can come up with "Flat Earth" is by living in the middle of a continental landmass.
Yup, that is still true for humanity; what's changed in the last few hundred years is trains, cars, airplanes, and them all becoming broadly accessible to people.
Still, that was then. Today, "flat Earthers" are mostly just peer groups of shitposters or extreme contrarians.
It varies somewhat by continent, but living very far from the ocean is what’s new. Humans have historically had by far the densest population near shores - river deltas, archipelagos, and so on.
The notion that “seeing the ocean” was a very special thing to most people in history is unlikely. To a Hungarian peasant or a Mongol shepherd, sure, but there were far more people along the Mediterranean coast, the Pearl River delta, and so on.
The reason is very simple: Ocean = free food.