Here's why:

* Gravity is a fundamental force: It's determined by the mass of the Earth and your distance from its center.

* Earth's shape: While the Earth is not perfectly spherical, its shape doesn't significantly affect the force of gravity at the heights of the stratosphere.

* Stratospheric altitude: The stratosphere ranges from about 10 km to 50 km above sea level. Compared to the Earth's radius (6,371 km), this change in altitude is very small.

So, while the density of the atmosphere decreases significantly in the stratosphere, the force of gravity remains nearly constant.

It's important to remember that *weight* is different from *gravity*. Weight is the force exerted on an object due to gravity. While gravity remains constant, the *weight* of an object in the stratosphere would be less because of the lower air density and thus less buoyant force.