When a ball rolls down an inclined plane, it gains speed because of gravity. When rolling up, it loses speed because of gravity.

Why doesn't gravity play a role when it rolls on a horizontal surface?

Because the path of the ball is perpendicular to the gravitational force.

Explanation:

In the first case, trajectory of the ball has a component parallel to gravity. Therefore, gravity speeds up the ball.

In the second case, trajectory of the ball has a component anti-parallel to gravity. Therefore, gravity slows down the ball.

When ball rolls on the horizontal surface, the trajectory of the ball is perpendicular to the gravitational force. Therefore, gravity doesn't play any role.