Here's why:

* Earth is the only planet with an atmosphere that supports human life. It has the right combination of gases, including oxygen, nitrogen, and carbon dioxide, along with a suitable temperature and pressure.

* Mars has a very thin atmosphere, mostly composed of carbon dioxide. It lacks enough oxygen and has a low atmospheric pressure, making it impossible for humans to breathe without special equipment.

* Venus has a dense, toxic atmosphere, primarily composed of carbon dioxide. The pressure is extremely high, and the surface temperature is hot enough to melt lead.

While there are other planets and moons in our solar system with atmospheres, they are not breathable by humans due to the presence of toxic gases, extreme temperatures, and/or pressure.