Here's why:

* Gravity's Pull: Earth's gravity exerts a constant pull on all objects, including the gases that make up our atmosphere.

* Atmospheric Density: The air near the Earth's surface is denser than air higher up. Gravity pulls more strongly on the denser air, keeping it close to the ground.

* Escape Velocity: For a gas molecule to escape Earth's gravity, it needs to reach a certain speed called escape velocity. Most atmospheric gas molecules don't have enough energy to reach this speed.

While other forces like the Earth's magnetic field and solar wind play a role in the atmosphere, gravity is the dominant force that keeps the atmosphere bound to the planet.