Here's why:

* Mass and Gravity: Gravity is a force of attraction between any two objects that have mass. The more mass an object has, the stronger its gravitational pull.

* Earth's Mass: The Earth has a huge amount of mass, making its gravitational pull very strong.

* Center of Mass: The center of mass of an object is the point where all its mass is concentrated. For a sphere like the Earth, the center of mass is at its geometric center.

So, because of Earth's immense mass and its spherical shape, everything on Earth experiences a gravitational pull towards its center.