Here's why:

* Gravity: The immense gravity of these celestial bodies pulls matter towards their center. This force acts equally in all directions, leading to a roughly spherical shape.

* Rotation: Planets and larger bodies also rotate. This rotation causes a centrifugal force that pushes matter away from the axis of rotation. This force is strongest at the equator, causing the body to bulge at the equator and flatten at the poles, resulting in an oblate spheroid shape.

Examples:

* Earth: A classic example of an oblate spheroid. The Earth's equatorial diameter is about 43 kilometers larger than its polar diameter.

* Jupiter: The largest planet in our solar system, Jupiter is also an oblate spheroid with a noticeable bulge at its equator due to its fast rotation.

Note:

* Smaller bodies: Smaller celestial bodies like asteroids and moons may not be perfectly spherical or oblate spheroids. They can have irregular shapes due to their weaker gravity and less significant rotational forces.

* Extreme cases: Very rapidly rotating bodies, like some pulsars, can have extremely flattened shapes.

In summary, the shape of dense bodies the size of planets or larger is primarily determined by the balance between gravitational forces pulling inwards and centrifugal forces pushing outwards due to rotation. This balance typically leads to an oblate spheroid shape.