Here's why:

* Gravity's Pull: Earth's gravity pulls on all the gases in the atmosphere, keeping them close to the planet.

* Gas Density: The air near the Earth's surface is denser than the air higher up. This density difference also contributes to the atmosphere's stability, as the denser air below pushes up on the less dense air above.

* Atmospheric Pressure: The weight of the atmosphere pressing down on the Earth's surface creates atmospheric pressure. This pressure helps to hold the atmosphere together.

Think of it like a balloon. The air inside the balloon is held in place by the elastic material of the balloon. Similarly, the atmosphere is held in place by Earth's gravity. However, gravity's pull isn't the only factor.

Other factors:

* Earth's magnetic field: The Earth's magnetic field protects the atmosphere from the solar wind, which is a stream of charged particles from the Sun.

* Thermal energy: The Earth's atmosphere is constantly in motion due to the uneven heating from the Sun. This motion helps to distribute the atmosphere's gases and keep it from escaping into space.

Together, these factors create a stable and dynamic system that we call the atmosphere.