Here's why:

* Gravity: Gravity is a force of attraction between any two objects with mass. The Earth has a large mass, so it exerts a strong gravitational pull on objects near its surface.

* Acceleration: Acceleration is the rate of change of velocity. When an object falls freely, it's constantly accelerating towards the Earth's center due to gravity.

* Standard Value: The value of 9.81 m/s² is an approximation and can vary slightly depending on factors like altitude and latitude. However, it's considered the standard value for calculations.

Important Note: The acceleration due to gravity is a vector quantity, meaning it has both magnitude and direction. The direction is always towards the center of the Earth.