Here's why:
* Gravity is a force of attraction between any two objects with mass. The Earth has a lot of mass, so it exerts a strong gravitational pull on objects near it, including you.
* This pull causes objects to accelerate towards the center of the Earth. Acceleration means increasing speed.
* As you fall, gravity constantly pulls on you, causing you to go faster and faster.
Of course, there's a limit to how fast you can fall due to air resistance. This is why you reach a terminal velocity, a maximum speed you won't exceed during freefall.
