Why are all planets round in shape?

 All planets are round because of gravity. When our Solar System was forming, gravity gathered billions of gas and dust pieces into clumps that grew larger and larger to become the planets. The force of gravity pulled this molten material inwards towards the planet's center into the shape of a sphere. A planet's gravity pulls equally from all sides. Gravity pulls from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere, which is a three-dimensional circle.

A smaller object like an asteroid doesn't have the same mass. In fact, the asteroid belt is a failed planet. There isn't the gravity to pull the material together into a spherical shape. It's rubble leftover from planet building, probably because there wasn't sufficient gravity there to pull a planet together and hold it together, up against the gravitational tugs of all of the other planets forming in the solar system.

When it comes to other objects like Meteorites ( much smaller rocks or particles in orbit around the Sun. ), they are almost never perfectly round or spherical. They are usually very irregular in appearance and come in a variety of different shapes and sizes.