Question

why are moon called satellite

Answer 1

A satellite is  defined as an object revolving around a planet. Since the moon orbits the Earth, a planet, it qualifies to be called a satellite. Since it is natural, it is a natural satellite. As simple as that. Compare this with say, Sputnik or some such man-made object that revolves around the Earth too.

Answer 2

because it revolves around a planet (earth),small in size that it can not be put in category of planets,does not have atmosphere,low gravity.Its a natural satellite not man made as you know..