it is round because from space, it is a sphere. It might not seem like it from our point of view, but the gravity earth has makes us think that Earth is flat, but it is round. For example, if you walk on Neptune, which barely has any gravity, you could clearly see a round planet from space and from where you are standing (or floating). It is gravity that is the main cause of us thinking that the earth is flat, but it is round. Gravity is pushing the earth down, creating a sphere. I hope this information was useful.