Hey there! Thanks for this post! I hate flat Earthers :P kidding
The simple reason why the Earth cannot be flat is Gravity. Gravity wouldn't pull objects vertically unless they were situated on the center of a flat celestial body. That basically means ships would sail from the edges towards the center without consuming energy and planes would need a lot more energy to travel in this direction than in reverse. Oceans would constantly flow towards the center of this flat Earth, buildings situated on the edges would demand entirely different technical design than those in the middle of a flat celestial body, trees on the edges would grow horizontally due to gravitropism and so on. The natural creation of a flat celestial body is impossible anyway.