There are 170 billion galaxies in the observable universe. Each galaxy has billions of stars and planets. With telescopic cameras and photos also taken from satellites sent into space (some having left our universe to continue beyond it) every single photo shows all the stars and planets to be like the shape of balls. Not one single star or planet has been observed to be flat, or cube shaped, or triangular shaped, or like a rhombus or any other shape you care to mention.
If the entire universe we know has ball shaped objects (even the gaseous planets), why would anybody think planet Earth is the one exception? Even when Voyager sent back a photo of what lay behind it (photographing the route it had taken from Earth) and our galaxy was pin-pointed, with planet Earth a mere dot within it, it was a round dot - like all the other star and planet dots. The Earth is shaped like a ball (or, perhaps slightly more like an egg but I'm not going to get technical.) It's certainly not flat.