Here's why:

* Gravity's pull: The Earth's gravity exerts a strong pull on all objects, including the gases that make up the atmosphere. This pull is what keeps the atmosphere bound to the Earth.

* Decreasing density: As you go higher in the atmosphere, the air becomes less dense. This is because the weight of the air above pushes down on the air below.

* Pressure gradient: This decrease in density creates a pressure gradient, with higher pressure near the surface and lower pressure at higher altitudes.

* Equilibrium: The balance between the force of gravity pulling the air down and the pressure gradient pushing it up creates an equilibrium that keeps most of the atmosphere relatively close to the Earth's surface.

The 32 km mark is significant: While there is some atmosphere beyond this point, the density is so low that it's considered negligible. This 32 km mark is roughly the boundary of the troposphere, the lowest layer of the atmosphere, where most weather occurs.