Why planets and stars are spherical?

All celestial bodies, including stars, nebulae are formed which in turn are huge clouds of gas and dust.

Arise many points of greatest condensation of gaseous matter and dust, creating gravitational centers are attracting more dust and gases.

They come to withstand high pressures and temperature and to come to possess a "critical mass" start thermonuclear reactions leading to the formation of stars. When the critical mass is not reached, the planets originate.  Planets and stars are spherical because of the "gravity" which originates from its center.

Other bodies not reached critical mass, they take irregular shapes such as Mars satellites Deimos and Phobos.