But how do you know it is not flat? Just because someone told you so? Just because you've read some things and seen some pictures?
Even in the ancient days, those living near the sea would know the Earth wasn’t flat. I had close to twenty years at sea, and the curve of the Earth is noticeable. There is also the little matter of what is quite easy to observe – the tops of a ship’s mast come into view before the ship. The tops of hills and mountains come into view before the shore. Even a lowly seaman would realize this is due to curvature.
Kinda makes ya wonder how anyone could ever believe in a flat earth, but not all people have had the benefit of wide travel as you have.