Here's why:
* Gravity's Pull: Every object with mass exerts a gravitational pull on every other object. The Earth, being a massive object, has a strong gravitational pull that attracts everything towards its center. This includes the atmosphere, which is held close to the Earth by gravity.
* Atmospheric Pressure: The atmosphere itself is a layer of gases, and the weight of all those gases pressing down creates atmospheric pressure. This pressure acts as a force that prevents things from escaping the Earth's pull.
Think of it this way: even if you jump up, you don't just float away. Gravity pulls you back down to the Earth's surface. The atmosphere experiences the same force, keeping it from dispersing into space.
While it's true that the Earth's atmosphere is constantly losing some molecules to space due to thermal escape, the gravitational pull is strong enough to retain the vast majority of the atmosphere.
