Answer :

SchwarzschildRadius
Well,

Large astronomical bodies are round because at their formation, gravity acted as the force to bring all of the matter together to form the body.  Since the gravity was acting from a point and bringing everything toward it, the final shape of the mass is spherical.  Non-round bodies are mostly that shape because other bodies "modified" the landscape.
benlovespi
They are round because of their enormous gravity. It pulls everything towards it strongly enough that every part of the body is the same distance from the center.

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