Earth is round thanks to one force, gravity. Well, it’s the interrelation of the mass of the world with the attraction of gravity. Everything has a gravitational pull, even a single piece of toast or a car. Anything that has mass has a gravitational pull. The bigger the object and the bigger the mass, the stronger the pull. Imagine the thing a size and weight of a planet, then we have the Earth.
Now go back 4.6 billion years ago, when the solar system was still a cloud of dust and gas. As gravity pulled together matter into itself, it spins, chunks up and stick together. These form the sun, moon and all the other planets and space debris. The core of the Earth is the first to form since the denser materials are attracted stronger closer to the source, sinking and binding. The lighter, less dense pieces form the crust. Slowly, the mass will add up and the gravity forces increase. The mass would take the most efficient shape, which is the sphere. The corners would be too far from the center if Earth was a cube. Since gravity acts on equal strength on everything, these corners would be pulled to the center. As a result, the surface is even, in general, with most points of the Earth at the same distance to the center. Spherical shape is the answer. Gravity becomes the main reason why it’s round.
However, Earth is closer to an ellipsoid but just looks round due to its sheer size. This is the result of the centrifugal force, Earth is wider at the equator than the two poles, about 70,000 feet wider. Earth rotation around its axis also maintain the reasonably ball shape. If it is still difficult to see why Earth should be round? Picture the following: a bread dough on a tray covered in flour. As you roll the dough on the tray more and more, the dough will take a round shape and as it is taking in more flour with each roll, it will become bigger. That’s pretty much how the Earth is now a ball, a rotational force of the rolling coupled with the gravitational pull of the sticky dough onto the tray of flour.