Because the of the Earth is pulling on air molecules and
pressing it against the ground, our
is thinner and thinner
the higher up you go.

Because the gravity of the Earth is pulling on air molecules and pressing it against the ground, our atmosphere is thinner and thinner the higher up you go.

Explanation:

Earth is a planet that has an active core, thus it has a reasonably strong gravitational pull. Gravity manages to keep all things on the surface of our planet and the ones that are in the atmosphere to pull them down or keep them near. This affects the atmosphere as well.

The atmosphere is composed of gases, all of which are made out of atoms and molecules. They are very light, so Earth is pulling them towards it. This has resulted in a situation where the majority of the gases are close to the surface, in the lowest parts of the atmosphere, and as we go up there is less and less of them, so the atmosphere is thinner and thinner as the elevation increases.