Here's why:

* Moons are formed through various processes: Some moons form from the same disk of gas and dust that formed the planet, while others are captured by the planet's gravity from elsewhere.

* Planetary size and gravity: The size and gravitational pull of a planet affect its ability to capture and hold onto moons. Smaller planets with weaker gravity are less likely to have moons.

* Orbital dynamics: The gravitational interactions between planets and moons can be complex. In some cases, moons can be ejected from their planetary systems.

Here's a breakdown:

* Planets with moons: Earth, Mars, Jupiter, Saturn, Uranus, Neptune

* Planets without moons: Mercury, Venus

So, while some planets have multiple moons, others have none at all.