Here's why:

* Space is a vacuum: Outer space is mostly empty, with very few particles present. This means there's almost no air to breathe.

* Atmosphere is a layer of gas: Atmospheres are layers of gas that surround planets and other celestial bodies. These gases are held in place by the gravity of the object.

* Lack of gravity: Outer space has very little gravity, so there's nothing to hold an atmosphere together.

However, there are still some traces of gas in outer space:

* Interstellar medium: This is a thin, diffuse gas that exists between stars.

* Solar wind: This is a stream of charged particles that emanates from the Sun.

These traces of gas are incredibly thin compared to Earth's atmosphere, and they don't provide the same protection or conditions that an atmosphere does. So, while there might be some particles in outer space, it's not considered an atmosphere.