Why all celestial objects (except asteroids) are all in spherical shape?

Gravity is the cause. It pulls everything together. Asteroids have such small amount of material that the total gravitational pull is not strong enough to crush it into a ball. The Earth does have enough pull for that but even so, not enough to completely round it out.

Our Earth is extremely large. The hills and valleys are tiny in comparison. So tiny in fact that those features are ignored whenever the shape of the Earth is discussed.

You may have a 12-inch globe of the Earth at home. Most globes have raised areas to show the mountains. If these features were actually to scale, you would not be able to feel them with your fingertips, even the highest peak, Mount Everest! At this scale, if Mount Everest dropped in altitude from its peak directly down to sea level (it doesn't), it would feel like the thickness of a single sheet of paper.

A neutron star is far more massive than our sun with a surface gravity so strong that it has no hills or valleys. The entire surface is smoother than a sheet of glass.
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It has to do with three things: gravitation, heat and plastic flow.

As a planet or large moon forms, gravity pulls radially towards the center of the body. If it collects enough mass gravity is so string that the crushing force heats the material until it becomes soft and plastic. It then takes on a spherical shape analogous to a drop of water becoming spherical (though the processes are very different). So, if the mass of a body is high enough, it will soften and become spherical in shape. A part of the heat internal to the earth is the action of gravity still crushing the material of the earth.

Smaller bodies, however, can cool off after undergoing this plastic flow. The moon is a good example as is Mars.

HTH

Charles

Most large celestial objects are found to be spherical in shape for two reasons. The first reason is because of gravity. The second reason is because of angular momentum.

The larger the celestial object, the higher its surface gravity. And the higher its surface gravity, the more likely a large protrusion will be crushed under its own weight.

Therefore, the larger the world, the more spherical it will be.

Gravity pulls on each planets, stars, etc material equally inward toward the center of mass or center of the object. Imagine making a snowball or crushing up paper, the snow ball or crushed paper is usually spherical because you push on all sides of the paper or snow equally, it's easy to understand but hard for me to explain with just words.

Now meteors and asteroids don't have as much mass as planets and stars so that means they don't have as much gravity either, so an asteroid or meteors gravity can't perfectly pull all the the rock and material together in a perfect sphere. Also i have read that asteroids sometimes collide with each other and the that causes chunks to break off causing irregular shapes. But some massive asteroids such as Ceres and Vesta are somewhat spherical.

*Please note that planets and starts aren't perfectly round, as they spin centrifugal force causes the sphere to bulge out around the equator.
Hope that explains it somewhat.

Very simple, they just don't have enough gravitational force pulling on itself to be spherical. That's why they have an irregular shape.

Gravity acts equally in all directions.

(though it's not actually spherical but elliptical)